A Treatise on Geology/Chapter 10

by John Phillips (1800-1874)

CHAP X.

STATE OF GEOLOGICAL THEORY.

ALL branches of the study of nature, in their progress from the period of observation to that of generalisation and theory, appear destined to endure the same storm which astronomy has weathered; and, like that noble science, to come forth renewed and purified in the struggle; strengthened by popular applause, and fertile of public benefit.

To quicken the inertness of prejudice, and rouse the despair of ignorance, among the masses of mankind, may appear unnecessary for the "advancement" of science, which must ever be in trusted to a few superior minds; but the opinion which would separate the acquisition from the diffusion of knowledge is no less erroneous than ungenerous, since the highest and most comprehensive truths in natural science are but the concentration of common phenomena, the laws of common experience. In the determination of these phenomena, in the correct association of them into laws and systems, immense preliminary labours must be undergone before the most powerful intellect, however deeply versed in abstract science and the philosophy of causation, can ascend to that comprehensive view of a whole series of dependent truths which constitutes a general theory.

Perhaps no term of importance in estimating the state of science is employed in more various and inconsistent senses than this word theory, which few branches of human knowledge have ventured to claim, but which is actually used as a term of reproach by men entirely ignorant of them. When correctly used, with the masters of Inductive Philosophy, it signifies the high point to which every faithful inquirer after truth in advancing, however slowly, in his peculiar branch of study; it is the unchangeable summit of a cone whose base continually enlarges to include every known fact appertaining to the subject; and whose every part is linked in harmony according to one simple and intelligible principle. Science is perfect only when it is in the form of a truly general theory; and perhaps it is not too much to believe that the utmost efforts of the human mind may fail in the attempt to comprehend all natural phenomena perceptible by our organisation in one such theory.

Even if, hereafter, it should be found possible to include the most comprehensive theory of ponder able matter, gravitation, and the undulatory theory of light and heat, into one wider generalisation for all inorganic matter, there would still remain the mysterious phenomena of life; and beyond all these the relation of mind and sensation. Now these are a few of the legitimate branches of the study of nature, which the providence of the Almighty Creator of the universe has committed to human reason. Their development is for man a physical revelation, continually enlarging its power and influence on the mind and heart; yet it leaves, almost without touching, except to support, a large circle of moral and religious truths of yet higher importance, and more lasting and powerful interest.

Geology dares not claim, as yet, the possession of a sound and general theory, such as is here described; but in common with astronomy, and chemistry, and mechanics, and every ether part of natural science, its infancy was amused with baseless speculations, and hypotheses which have fallen into contempt. For these errors of their fathers its modern cultivators have dearly paid, and fairly atoned. The wanton and ignorant abuse lavished on the magnificent problems to which their lives are devoted, has been endured with patience; the principles which have guided other and easier branches of philosophy in their successful progress, have been wisely copied; they have begun at the foundations of the temple of truth; they have collected an inconceivable number of individual facts; they have combined these into correct, though incomplete, generalizations; and have called on zoology and botany, en chemistry and mechanics, to furnish the interpretation.

Geology has thus been placed, by the energy and prudence of its living advocates, in the circle of inductive science; no more to be dissociated from the ether parts of knowledge; advancing with them, and often leading them forward, by the proposal of new and remarkable problems, to the solution of which all the collected resources of modern science are sometimes scarcely equal. In this career the Geological Society of London has proceeded, without faltering, for thirty years, and the reward of their labours is in the just and candid acknowledgment of one most competent to pronounce, that "Geology, in the magnitude and sublimity of the objects of which it treats, undoubtedly ranks, in the scale of the sciences, next to astronomy."^[1]

If the object of this treatise were to produce merely the entertaining parts of geological discussion, it might be very proper to introduce a notice of the many fanciful and absurd systems of cosmogony and philosophy, falsely called "theories of the earth." Perhaps, notwithstanding the discredit which such mistaken attempts have brought upon philosophy generally, rather than geology in particular, some useful result might be derived from a dispassionate survey of them. For if Woodward, Whiston, and Burnet,—Buffon, De Luc, and Werner, have failed in the great attempt to unveil the natural history of the earth, it was not so much because of any inferiority of intellect, want of patient research, or deficiency of information, that their "theories" have fallen into oblivion; but because the process of induction, without which no true theory can arise concerning the works of nature, was not at all, or imperfectly followed out.

There has been, moreover, from early times, in consequence, perhaps, of an imperfect apprehension of the nature and object of revealed religion, as compared with the physical truths which are left to the discovery of human reason, a singular propensity to supply the deficiency of philosophical research by arbitrary appeals to the authority of scripture. The danger to religion of such a reckless course is too well understood by the enlightened theologians of this age to render more than a passing remark necessary; though, even in the nineteenth century, it occasionally happens that astronomical truth is questioned, because the scriptures, addressed to an unlearned people, speak popularly of the sun "standing still;" and the established inferences of the successive revolutions in the state of the globe, which are not mentioned by Moses, but which invest with new interest the study of the ancient earth, are thrown aside in favour of some physical and theological absurdity, such as that which makes the stratified crust of the earth the effect of one tumultuous flood, and turns the "fountains of the great deep" into submarine volcanos, or hides a world of waters within the globe.

The mention of these unhappy errors would be painful, could we believe that the progress of pure religion or sound philosophy could be checked by their influence. Let it be remembered that the Bible teaches no physical science, and that philosophy has made little progress in physical truth, if it does not recognise among all the multiform changes of the universe the power and the will of One Supreme. From this highest point of true philosophy, as from a sure foundation, a pure religious faith must spring. Of the importance and independence of physical truth none of the distinguished ornaments of the Christian faith, from St. Augustin to Boyle and Chalmers, have been ignorant; and to their immortal works we beg to direct the attention of those inconsiderate persons who think to advance Christianity by denying philosophy, and to confirm revelation by making its very truth depend upon their own narrow interpretations of nature.

Lest, however, we should fall into as great absurdities of another kind as these we have mentioned, it will be prudent to determine, if possible, the true character of a general theory of the earth; for in this there is a great liability to error. Geology, regarded as a body of facts, comprises not exclusively, nor specially, the phenomena which are now, or have been at any one former time, in progress on and within the earth, but embraces the whole succession of these occurrences, from the earliest operation of natural laws on the globe to the present hour. Each of the phenomena, taken singly, is the subject of interpretation by some special branch of natural science: the characters of organic fossils are referred to the zoologist and botanist; mineral compounds are examined chemically and crystallographically; the fractured crust of the earth receives explanation from the application of mechanical philosophy. The general view of these and other phenomena, manifested at one epoch, or during one period, and the survey of the condition of the globe at several such periods, are the proper objects of geological observation; and the successive states of the globe being thus ascertained, it is the business of inductive philosophy to discover the general antecedent condition or proximate cause upon which these successive states depend. If the research be successful, the result is a general theory of the earth; that is to say, a sufficient natural cause is found to explain, in combination with other agencies really existing, all the characteristic changes which have been observed in the earth's condition, in the degree, combination, and sequence which actually belong to them.

Perhaps an illustration may be usefully taken from exact science. la mathematical inquiries, a particular result or condition of things is frequently capable of distinct representation by means of a series of quantities, unequal in value, and combined in different proportions; yet the origin, formation, and succession of this series of dissimilar combinations of unequal quantities may be perfectly simple, and often is perfectly known, though the series be demonstrably boundless in one direction. Now, in this case, the "theory" of that series is really known; and, in exactly the same sense, the "theory" of the series of dissimilar combinations of unequal phenomena, which succeeded one another in a certain order of time, upon and within the earth, appears attainable by the human intellect.

Every attempt to ascertain the law of succession among the phenomena manifested in the structure of the earth must entirely fail, unless, at least, the characteristic facts, and combinations of facts, belonging to each successive geological period be completely known, and these periods completely defined. It is therefore necessary, before noticing the attempts which have been made to establish a geological theory, to ascertain how far these, indispensable preliminary conditions have been fulfilled. The historical view of the series of stratified rocks, contained in the first part of this work, will show to what extent the author has been able to reduce to rule and system the known phenomena occasioned by the action of water on the globe; in the succeeding part a similar attempt is made to unfold the parallel series of truths concerning the unstratified rocks, and other effects of heat. Though neither of these attempts ought to be taken as a measure of the progress made in similar inquiries by other individuals, and still less as a summary of the whole knowledge on the subjects, the intelligent reader will easily perceive that, with regard to the mechanical and chemical agency of water in depositing earthy sediments, and the changes of organic life on the globe at several successive epochs, the monuments of nature have been extensively collected, ranged in their real order, and in a great measure truly interpreted. Very large portions of the land and sea are however still unknown in this respect.

Hardly so much can be said regarding the effects of heat; for though these are for the most part clearly, they are not completely, interpreted, nor is the order of their succession sufficiently known. It appears, however, that the products and effects of heat at different times have not varied so much as those of water, so that the order of succession among them is of less importance.

In a complete geological theory, not only the order of succession among the several groups of phenomena would be deducible from the principles on which it was based, but also, in proportion to the completeness of the facts indicating the lapse of time, the real duration of the several successive geological periods would be at least approximately known. It would be very unwise to attempt this at present, because of the imperfection of the data in every part of the series of strata; and in this respect geological theory is not singular, for even the most perfect mathematical theorem is equally inapplicable to incomplete data. This was strongly felt by the geologists of England, who gave a fair proof that hypotheses were out of fashion, when they declined to compete for the medal which the Royal Society offered to encourage researches into the antiquity of the globe. (See on this subject of geological time, Vol. I. chap. i.)

It may perhaps be thought, that the limits which have been fixed for a legitimate "theory of the earth" are sufficiently wide to include an immense number of general speculations; and that many conflicting hypotheses, advanced by Neptunists and Plutonists, should now be compared and condemned. But, in truth, a little consideration will prove that there have not been, and can never be, more than two hypotheses really general on the subject. Nature, as we see it, exists under the influence of particular forces and conditions, vital, chemical, and mechanical; and the sum of the phenomena that now occur in a given time is the measure of those forces and conditions. The exterior influence of the sun, and the ethereal spaces; the mass and quality of the atmosphere; the size, figure, density, and motions of the earth; the distribution of land and sea, —are all circumstances of great importance, to which the vegetable and animal productions of the globe, as well as the chemical and mechanical operations upon it, are adjusted.

It is soon apparent to the inquiring mind, that many of these conditions and forces vary, and with them, from time to time, suddenly or gradually, the characteristic phenomena of life and inorganic matter. If we knew the measure of these variations, the real state and momentary condition of the earth at the present, in former, and in future periods, would become a practicable problem.

Now it must be evident that we have not such knowledge; for the variations in question, though quite sensible, are too complicated to be understood, except through an immensity of recorded observations; and of these we have few that are trustworthy, except in astronomy. In astronomy, with the help of the general theory, it has been found possible to determine the "limits of variation" due to the disturbing forces of the planetary system; but it is impossible to effect this in geology, from a survey of existing nature, for want of such a theory. Incapable, therefore, of learning from the most perfect survey of nature as it is, whether terrestrial phenomena are subject to progressive and permanent changes, or to a limited circle of compensating variations, the leaders of geological speculation have assumed one or other of these views—the only really general ones which the subject permits; and thus we have, on the one hand, Leibnitz deducing the principal geological phenomena from the gradual refrigeration of an ignited globe; and, on the other hand, Lyell, and the followers of Hutton, maintaining the sufficiency of "modern causes," acting with their present intensity, to account for all, even the earliest traceable changes of conditions of our planet. The real distinction between these celebrated speculations consists not in the nature of the physical agencies which are assumed to have accomplished geological revolutions—for there is little difference, in this respect, between Playfair and Leibnitz, Lyell and De Beaumont—but in the measure of intensity assigned to them in different geological periods. In both, the same laws of material action are invoked, the same causes are recognised in their effects; in both, the combinations among these causes are admitted to vary locally and periodically; both contemplate periods of immense duration as necessary for the production of observed phenomena. But in one, the Leibnitzian "theory," the globe is supposed to have undergone a general and progressive loss of interior heat; in the other, to have experienced only local or periodical variations of surface temperature; in one, great and general revolutions in the condition of the globe are deduced from a gradual refrigeration of its substance; in the other, general revolutions, properly speaking, have no place, but local changes, and new combinations, arise in endless succession: in one, the mechanical, chemical, and vital phenomena must necessarily proceed with an entirely different rate of progress, in different geological periods, because the powerful influence of heat was continually changing; in the other, these phenomena exhibit an undeviating general uniformity, such that "equal effects are produced in equal times." Taken on a great scale, time, in arithmetical series, is the element of a cycle of variations in one hypothesis; the product of time and force (one increasing as the other decreases in geometrical series) is the principle of continual progression in the other.

To enter fully into the consideration of these rival hypotheses would be at present fruitless; but we may try their power and truth on some of the more important and fundamental points in the structure of the earth, such as the actual physical geography, and the ancient climates of the globe.

Physical Geography.

Distribution of Land and Sea.

No truth is more certain or important in geological reasoning than the formation of all our continents and islands by causes acting below the sea. As far as relates to the stratified rocks this is obvious; but it is not the less certain for the unstratified rocks, these having undoubtedly been uplifted to our view from beneath the strata. It is possible there may yet be found some granitic rocks which were raised above the general spherical surface before the production of any deposits from water, and which may therefore be presumed to form an exception to this general rule; but such truly "primitive" rocks have nowhere been seen, nor is there any ground of expectation that they will be discovered. The elevation of the dry land out of the sea is therefore one of the great truths to which we must compare general speculations; and it affords a test, and prescribes conditions, which no false "theory" can fulfil.

The actual distribution of land and sea is very remarkable. London being taken as the centre of a hemisphere, nearly all the land is included therein. The antipodal hemisphere includes a vast abundance of small islands; but there are no considerable antipodal surfaces of land, except where Chili and Patagonia oppose the eastern part of China, and the volcanic islands of Sumatra, &c., oppose the volcanic mountains of Quito. The continent of Australia is opposite to the deep centre of the Atlantic Ocean. Only $\scriptstyle {\frac {1}{27}}$th part of the present continents and islands has land opposed to it.^[2]

The meridian of least land (about 16° W. long.) passes by Kamschatka, the east side of Hecla, the west coast of Africa (near Madeira. Teneriffe, the Cape de Verde islands), the west side of New Caledonia, and near the west side of New Zealand. On this line it is nearly all sea. The distribution of land and water presents little symmetry; yet a meridian at right angles to that above noticed leaves, east and west of it, nearly equal masses of land. The poles are believed to be situated in the midst of extensive oceans, though the progress of modern research has augmented our knowledge of antarctic land. These circumstances, though they indicate little of symmetry in the rugged and irregular surface of the globe, supply some points not unimportant. The general spheroidal figure of the earth is obviously not the result of superficial waste and minute arrangements, as the hypothesis of the invariability of natural forces would seem to require; on the contrary, this figure appears clearly due to the general conditions of the interior masses, which are only marked and rendered irregular by the changes that have happened at the surface. Upon the Leibnitzian supposition, that the crust of the globe is cooled over an ignited nucleus, which is still further undergoing refrigeration, it appears possible to understand the accumulation of water about the poles, since, in the direction of the polar diameter, the contraction of bulk would be in no degree balanced by the augmented centrifugal force, corresponding to a determinate velocity and a diminished diameter. But, on the supposition of the spheroidal earth having been originally a sphere, and having derived its actual figure from superficial processes, the polar regions should have been very elevated land, and the equatorial zone deep sea. This neither is, nor appears to have been, the case.

Again, the remarkable contrast of a hemisphere of land opposing one of sea marks very clearly the influence of some great and general alterations of surface level. The supposition of a cooling globe undoubtedly meets this case; but it appears difficult to see how the rival speculation can be applied to phenomena on so vast a scale, even if unlimited time be given to the operation of "modern causes."

Heights and Depths.

The elevations on the land rise at most to about five miles above the level of the sea; and the depths of the Atlantic may perhaps be justly estimated at nine miles, from the data furnished in Mr. Whewell's Essays on Cotidal Lines.^[3] The labour would probably not be wasted which should be given to a careful estimate of the mass of the sea, as compared with the mass of land raised above its surface; on the hypothesis of a gradual refrigeration of the globe, it is perhaps not impossible to determine by calculation the relation of these masses; and from a comparison of these independent results there would arise an important test of the truth of the speculation. The heights and depths of the land and sea appear to require the supposition of co-extensive upward and downward movements, and, as Mr. Lyell has shown, it is probable the depressions exceeded the elevations. These effects appear unintelligible, except upon the admission of subterranean surfaces of melted rock, capable of yielding to subsidence inward, and eruptive forces outwards.

But this conclusion becomes more decided when we take into account the continuity of mountain chains and oceanic depths, the abrupt borders of the sea-coasts, the large areas of tertiary and secondary strata which were formed in the old sea bed, and are now raised, with little mark of local violence, into almost boundless plains and vales, within a border of bold mountains. All these circumstances are the natural consequences of extensive depression of the crust of the globe, followed by elevations; both being determined in greater intensity to points, lines, and areas of weakness, in a solid crust above a fluid of small compressibility, like melted rock.

Displacements of Stratified Rocks.

The notices in a former chapter (Vol. I.) will probably suffice to satisfy the inquirer after geological truth, that the elevation of stratified rocks to their present height above the sea is not merely relative, not merely caused by great depressions of the earth's surface elsewhere, but, in part at least, dependent on a real uplifting of mountain chains and other groups of dislocated strata. The most obvious argument in support of this is the well-known fact, that, in approaching the mountains, three orders of phenomena rise together to importance; the inclination of the strata becomes more and more decided and violent, till they appear vertical or even reversed; the marks of violent displacement augment in a corresponding degree; and the exhibition of igneous rocks becomes continually more frequent among the fractured and contorted strata. Now, if the mountain lines and groups had been points of rest, while all the spaces round them sank, something like the present distribution of land and sea would have appeared, but these signs of violent displacement would not have predominated in the vicinity of the mountains. There is no doubt, therefore, that these have been local centres of violence and not of rest.

The elevation of mountains has been too much regarded in the light of an insulated phenomenon: Mr. Darwin has truly pointed out its relation to continental elevation, which may be regarded as the great effect of a general cause manifesting itself at particular points in greater intensity. Just as, in experimental pressures, on solids of every form, the weakest part alone yields to a force which, up to a certain point, was borne equally by all, we may easily conceive a general continental elevation to a certain point, but beyond this, a partial rupture and relief of the pressure along a particular fissure. This is Mr. Darwin's view of the phenomena of uplifted land in and on either side of the Andes.

The same eminent observer has applied the same consideration of extensive displacements of land and sea to explain the alternate bands of elevation and subsidence, which are inferred from his survey of modern coralligenous reefs and islands. (See Vol. I. p. 336.) In this generalisation it appears that the points of volcanic eruption all fall on bands of general elevation, where the uplifting force is at a maximum. Volcanic action might thus suggest itself as the local cause of this local maximum of elevation, did we not know that exactly the same relation of continental and mountain elevation obtains for areas of land and groups of mountains where no volcanic eruptions have happened. (Nevertheless, it is not to be denied, that the effect of volcanos is, generally, to augment the inequalities of level of the earth's surface.) If this view of Mr. Darwin be well established, it will go far to confirm the general probability of a refrigerating globe; for movements of such regularity and extent require a corresponding slowly and powerfully acting cause, such as a general change of temperature must be acknowledged to be. "A change of the form of the interior fluid surface of the globe," as Mr. Darwin very correctly expresses the general condition on which all these phenomena of simultaneous elevation and subsidence may be made to depend, is a result strictly deducible from the hypothesis of a refrigerating globe; and the interesting examples of gradual and prolonged elevation in Scandinavia, and perhaps of subsidence in Greenland, appear natural and obvious consequences of that doctrine, while more violent upward and downward movements in other parts of the globe are not at all opposed to it.

The elevation of mountains is, in the doctrine of refrigeration, a local, critical, and more or less sudden result of a general and gradually accumulated force; the contrary hypothesis supposes a vast multitude of minor movements, such as earthquakes, which now happen in volcanic regions; and that these successively contribute their effects in one direction. The magnitude of single movements of the stratified rocks thus becomes a criterion of importance in estimating the value of these contrary views.

Anticlinal axes, such as that of Snowdonia, great faults, like that of the Penine chain, will perhaps he easily acknowledged to be absolutely unparalleled in historic periods; but this inequality of mere magnitude will not furnish a shadow of evidence against the application of the doctrine of the sufficiency of modern causes, unless it be proved, or shown to be probable, that the chain of Snowdon, and the ridge of the English Apennines, were thrown up by one, or, at most, a few efforts. Now this is probable in each case, for reasons based on observation, and, as will hereafter appear, not improbable for reasons founded in mechanical science.

Observation detects on the line of these great movements of the earth's crust no trace of the minutely confused and fragmentary condition of the strata, which must have been the result of an indefinite number of small convulsions, like those of the Chilian earthquakes in 1822 and 1835, when the ground rose convulsively a few feet; on the contrary, the simplicity and completeness of the anticlinals of Snowdon and the Isle of Wight, and the violent single fracture and few bold contortions on the Penine fault (which ranges for above a hundred miles, and may possibly extend much farther), speak of one or a few powerful efforts. This is so much the more to be trusted, as the effect of the friction on the surfaces of motion has the same character of simplicity. The area uplifted by the Penine fault may be roughly estimated at 2000 square miles; and the vertical extent of the movement may be taken, on the average, at 2000 feet. The Chilian earthquake, even if the ground were uplifted 4 feet for 100,000 square miles (neither of which assumptions seems at all supported by the narratives which are published^[4],) would yield, at most, only $\scriptstyle {\frac {1}{10}}$th part of this mass of land.

Direction.

The direction of anticlinal lines and other great dislocations of the strata has become of importance in a theoretical point of view, ever since Humboldt, Von Buch, and De Beaumont, strove to link these features of physical geography with particular epochs of geological time. If the parallelism of the Carnarvonshire and Radnorshire axes of movement is an indication of their being contemporaneous—and this analogy and conclusion can be extended to the primary slates of Cumberland, the Lammermuir, Isle of Man, and Grampian mountains—the inferences justly drawn from one district, as to the mechanism of its elevation, become confirmed in a very exalted degree. It is, therefore, most important to inquire, not merely what foundation there may be for the particular system on this head, which is supported by the learning and talent of De Beaumont^[5], but further, within what limits observation or mechanical science allow us to consider it possible to determine the geographical extent of contemporaneous disturbances of the strata.

The propositions of M. De Beaumont, in their utmost extent, may be thus understood. The principal dislocations of the same geological age range in lines parallel to one and the same great circle of the sphere; those of different ages are parallel to different circles. The geological era of the elevation of, mountains may be known from the direction of their axes of movement. The mode of proof will be understood from the following abstract relating to the system of dislocations, referred by De Beaumont to the period preceding the deposits of green sand and chalk; and the extension, by analogies, from a limited proof to a large range of probabilities, will appear in the short notice of two other systems of later date.

Three small granite eminences, in the Côte d'Or, near Sombernon, which have occasioned the disruption of Jura limestone there, range in a line N.E. and S.W. parallel to the summit ridge of the Côte d'Or. The line of these granite points being considered part of a geodesical circle, and prolonged in each direction, is found to coincide with several remarkable geological accidents or disturbances. In the N. E., for instance, it coincides with dolomitic oolite and steep dips at Sury, between Langres and Dijon; with the hot springs and magnesian muschelkalk of Bourbonne les Bains; with the basaltic eminence of Essey, of Luneville, and with the granitic protuberance of Albersweiler, between Annweiler and Landau.

Another line of disturbance, parallel to the preceding, is indicated; and it is observed that from Paray (Saone et Loire) to Plombières (Vosges), the great line of valley watered by the Bourbonne and Saone is perfectly parallel. This line, prolonged into Germany, passes along the valleys of the Mayn and the Saal, through Mittenberg to Leipzig, and is parallel to the Erzegebirge and Mittelgebirge.

Now all these dislocations were probably produced at the same geological epoch; which, though inferred from the general phenomena along the line, is determined more exactly in consequence of an extension of this system of faults by a series of parallels retiring to the S. E., till we arrive in the department of the Rhone, where the chalk and Jura limestones are found together—the latter dislocated, the former undisturbed. The direction of this line of disruption is N. E. and S. W. at Dijon.

In the Jura, a great number of undulations in the strata range parallel to a line N. 40° E., or N. 45° E.; and, being sometimes filled with green sand deposits, are clearly of the same date as the above disruptions.

In abstracting the proofs of the other grand systems of elevations, we shall attend less to the minute than to the general analogies. The insulated chain of the Pyrenees, one of the most remarkable in Europe, forms the base of the system. Many observations prove that the chalk and green sand are here uplifted with the primary rocks; but the later marine lacustrine deposits lie level upon their slopes, and were clearly deposited from a sea which washed the base of the already elevated mountains.

The general direction of the chain from Cape Ortegal in Galicia to Cape Creuss in Catalonia, is a little south of east; but this general chain is composed of partial ridges, whose axes are parallel to one another, and directed W. N. W. and E. S. E.

This direction belongs to the disturbances of the same date in Provence, and near Nice, and is recognised in the Apennines, at least in the northern part, and in the country of Naples, and along the south shore of Sicily. The south western boundary of the Nägelflue in Switzerland appears to correspond with the Pyrenæo-Apennine line; as do likewise the Dalmatian and Croatian summits, the valleys of the Save and the Drave, the line of the Rhodopian mountains, and the ridge which crosses the straits of the Bosphorus. Similar directions seem to be traceable in Greece; and, as far as the evidence yet collected goes, the date of the elevation of all these mountains may be the same. The Carpathian range, parallel to the Dniester, falls into the same system, with a small line of granitic and sienitic rocks along the Elbe near Dresden, and the mean courses of the metallic veins of the Hartz. Finally, the well known disturbances of the strata in Sussex and Hampshire have the same age, and lie in the same parallel. Extending his views, M. De Beaumont finds some traces of the Pyrenæo-Apennine system in Africa, and Syria, in the Caucasus, and in the Ghauts of India; but the imperfect state of information concerning the geology of these countries renders the inferences concerning them far from precise.

On prolonging the Pyrenæo-Apennine circle across the Atlantic, by Hecla and Greenland, to the New World, we find it descend parallel to the Alleghanies and their northern connexions, which have determined the form of the eastern shore of the United States, and, as De Beaumont collected from the statements of transatlantic geologists, were probably uplifted between the age of the chalk and the latest of the stratified rocks.

Such remarkable accordances of epoch and linear direction, over so enormous a length upon the surface of the globe, cannot, says De Beaumont, be the result of chance, but of a regularly acting internal cause.

M. de Beaumont has entered into a minute examination of dislocations affecting the molasse, one of the most recent of the tertiary deposits. He has connected the line of these disturbances in the south of France with those which may be observed in the western Alps from the Grande Chartreuse near Grenoble to the Salève near Geneva, and in the primary chain from the mountain of Taillefer to Mont Blanc, in the direction north, 26° east. Numerous observations in the valley of the Durance, though full of discordances, are reduced by the author to the same general line north, 26° east, which agrees with the opposite escarpments of Mont Blanc and Mont Rosa, and nearly with the line of a remarkable dislocation parallel to the Jura from Molezon to Aarburg, and with the depressed region occupied by the Lungern See, Sarner See, Alpnach, Kussnacht, and the lower parts of the lakes of Zug, Zurich, and Constance. The volcanic cone of Hohentwiel, beyond Schaffhausen, being upon the same line, gives occasion for the remark, that a system of disruption of the same age has thus been traced in one direction for above 100 leagues.

In the prolongation of this line to Nova Zembla, no instance is mentioned of corresponding disruptions; but the long Scandinavian Alps, and particularly the Dovrefeld Mountains, are parallel to it; and it was in consequence of their elevation that so large a quantity of Norwegian rocks have been scattered over northern Europe: the late date of this dispersion of blocks proves the late date of the elevation of these mountains.

Some traces are supposed to be found in Africa of the same line of disturbance, and even the chain of the south-east coast of Brazil, from Cape Roque to the Plata, though 400 leagues distant from the great circle of Zurich and Marseilles, might, perhaps, upon this hypothesis, be referred to the same epoch.

The most striking difficulty to the reception, at present, of any hypothetical connections between geographical lines and the irregular lines of disruption of strata, arises from the excessive number of these disturbances, and the variety of their directions. Brongniart has expressed, in strong terms, his impression on this subject, by saying that there is hardly a square myriameter of the earth's surface which has been left in its original position.

This difficulty, however, would only perplex the observer, not obscure the reasoning. There is another of more importance. The exact geological date of a dislocation of strata is very difficult to determine, and in most cases is merely known within wide limits. Who can prove the contemporaneity of the elevation of Snowdonia and the Grampians, when the strata dislocated are not the same, and the covering deposits are different? In the north of England the rothetodteliegende and magnesian limestone cover dislocated coal; in some parts of the south of England they are not traceable. The dislocations of the coal may be of the same age in both districts, but it is impossible to prove it.

These are difficulties in the examination of De Beaumont's views, not objections to their truth. There is, apparently, only one mode of discussion which is likely to be at all satisfactory: we may compare together the directions of dislocations, which are probably of the same geological period, and afterwards some of those which are known to belong to different periods.

The first class of dislocations, which, in this vague sense, may be called contemporaneous, belongs to the period anterior to the whole carboniferous and old red sandstone series of rocks. To this period the anticlinal axes of the Highlands and Lammermuirs, prolonged to Donegal and Cavan, the Cumbrian mountains, the Isle of Man, and North Wales belong. Now all these axes of elevation range north-east and south-west, and thus appear to support De Beaumont's hypothesis. Professor Sedgwick, in a recent communication to the Geological Society (May, 1838), speaks of the importance of attending to this conformity of direction in the axes of elevation, while attempting to join into one classification, according to geological age, the formations of distinct regions. He states further, in support of the same general views, the probable contemporaneity of the parts of another and later system of dislocations passing east and west in Cornwall, Devon, and South Wales, after the deposition of the coal strata. Lastly, he notices a system of dislocations which have brought up a portion of primary rocks, at Dudley, on both sides of the Coventry coalfield., and in Charnwood forest. At all these localities the "strike" is the same, and the lines of the greatest movement are nearly parallel, all being about N.N.W. and S.S.E.; and all these movements belong to one epoch, having been completed after the deposition of the lower new red sandstone (rothetodteliegende), and before the period of the upper sandstone and gypseous marls. Hence we have three great systems of elevation, which occurred during three distinct geological periods, and range in three distinct geographical directions.

This favourable testimony to the hypothesis of De Beaumont might perhaps be further extended: it is, however, met by the following facts:—

Dislocations almost perfectly parallel to those of Devonshire and South Wales range across the cretaceous and tertiary systems of Hampshire, Dorsetshire, and Sussex. In the counties of Radnor and Brecon, anticlinal axes range N.E. and S.W. through districts where the old red sandstone is conformed to the primary strata; and the same direction is observed extensively in the south-western part of Yorkshire, in anteclinals which cross the upper part of the mountain limestone series.

Here, then, dislocations of very different ages appear conformed in direction to some that have been mentioned before.

With such uncertainty in the data for reasoning and such contrariety and complexity in their indications, it is obvious that no definite and satisfactory conclusion can be at present adopted on the question of the parallelism of mountain elevations which belong to one geological age.

The great ranges of mountains, &c. marking the dislocations of the strata, cannot at present be accommodated to the strictness of a general geographical system; it is, however, not the less desirable to examine the same question on a smaller scale, with the aid of mechanical science and rigorous observations.

The well-established facts of the local parallelism of particular classes of mineral veins, already put in evidence in a preceding chapter, leave no doubt of the existence of some real symmetry of the systems of dislocation in every limited district. In several instances approximate parallelism has been observed between mineral veins and the numerous divisional planes of stratified rocks; and in others a peculiar dependence has been traced between the direction of a vein-fissure and that of an axis of elevated strata. Phenomena of this nature would for ever remain unexplained, if mathematical methods of research could not be applied to them; nor can they be applied except upon certain assumed conditions of mechanical action.

The first step in this career of discovery has been taken by Mr. Hopkins, whose memoir in the Cambridge Philosophical Transactions is remarkable for the simplicity and probability of its fundamental postulates, and the ready applicability of its conclusions to the results of observation. That the crust of the earth is elastic and capable of extension, earthquakes demonstrate; that cavities exist below parts of it is certain; and that these have a considerable horizontal extent is probable. There is no room for doubt that similar conditions existed in early geological times; for such cavities below the earth's crust would probably arise either from general refrigeration of the globe, or from local variation of heat. In such cavities the accumulation of elastic vapours is almost a necessary consequence, and it is conceivable that the crust of the globe would in parts yield to their force.

But Mr. Hopkins's reasoning would be in no degree invalidated, if for this mechanism of elastic vapours and cavities, an outward pressure derived from some other cause were hypothetically substituted, provided only that the area of its operation were sufficiently large, and its force continuously augmented until the earth's crust broke with the accumulated strain. The direction of the fissure at the instant of fracture can be determined mathematically, whether the intensity of the elevator force be uniform at every point of the surface, or greater at particular points; provided the boundaries of the surface and the resistance offered by the cohesive power of the mass raised and broken be known. This last condition, indeed, does not require to be very precisely fulfilled, except in a horizontal direction; for in a vertical plane, the cohesive power may vary according to any discontinuous law, as must happen in every series of dissimilar strata. (The pre-existence of joints in the rocks raised offers greater difficulty; but as few of these traverse great masses of rock, and each stratum has some peculiarity in the distribution of the joints, it does not appear to us necessary to except even this case.)

The following are among the results of the investigation when applied to a case resembling the actual condition of the stratified crust of the globe.

1. Production of longitudinal fissures.—If the mass of ground raised by an elevator force of uniform intensity be of indefinite length, and bounded laterally by two parallel lines, the extension and therefore the tension at any point will be in a direction perpendicular to the length; and the line of fracture will necessarily cross this direction, so that fissures cannot be produced under these circumstances, except in a longitudinal direction, or parallel to what may be called the axis of elevation. It appears that these fissures will not commence at the surface, but at some lower part of the mass. The whole series of stratified masses will be affected by the tension in the same manner, but under some conditions the fissures may not reach to the surface. The fissures will be of nearly uniform width at all depths, except that unequal elasticity in the dislocated strata will cause some differences. It is not inconsistent with mechanical principles to admit that more than one parallel fissure may originate simultaneously, and they may be subsequently prolonged, so that many parallel fissures (especially below the surface) may exist together, the fruit of one general action. No sooner, however, are the fissures extensively formed than new conditions arise, and any further fracture can be produced only in new directions. Wherever such a system of parallel fissures is found to exist in the same mass of strata, it is physically impossible that they can have originated at considerably different times, though the prolongation of a fissure may have been effected long after its origin.

2. Formation of transverse fissures.—In a district, circumstanced as stated, the application of any further force would cause extension of the now free parallel parts of the mass only in the direction of their length, and consequently produce ruptures at right angles to the former fissures. One or more of these transverse fissures might in like manner be produced in each of the parallel bands of displaced strata. In any country which manifests two systems of parallel fissures, one at right angles to the other, it is absolutely certain that the effects are due to no more than one general elevator force, and one continuous effect for each system of parallels; a series of partial forces at particular points or different times could not produce the effects.

3. Formation of fissures in a conical elevation.—If the mass of strata moved offer a uniform resistance, a conical elevation of a part can only be occasioned by forces of great intensity determined to a limited area. Fissures will in this case be formed so as to pass through the axis and radiate from the centre of the cone, as is observed to be the case in the Plomb du Cantal. If in addition to a general elevator force, supposed to act in the production of longitudinal fissures, a partial force was simultaneously acting at a particular point, the fissures would deviate from parallelism to approach that point. An instance of this was observed by Hopkins, in connection with a limited elevation of millstone grit, through the coal strata of Derbyshire.

4. Faults.—The masses thus separated by fissures might, upon the weakening of the elevator force, fall back in some confusion, so as to occasion faults of different kinds.

We shall only observe further on this subject, that a circumstance of importance in determining the direction of the lines of fissure is the weighting of the masses, which for many reasons must be supposed to have been often very unequal. The more general the mechanical agency, and the more uniform the resistance of the masses, so much the more perfectly straight and parallel the systems of simultaneous and successive fissures.

The conclusions thus obtained seem to apply with special accuracy to the veins and cross courses of Cornwall, Brittany, Cumberland, and Northumberland, the Hartz, the Erzgebirge, and other districts, and assist very much to strengthen the conviction derived from other phenomena, that the great faults and other forms of disturbance may have been occasioned by single continuous efforts of general subterranean forces. If so, it is difficult to believe they can have been due to such effects as those made by modern earthquakes.

Periods of Ordinary and Critical Action

Whatever may be the fate of De Beaumont's speculation regarding the elevation of mountain groups, at particular geological æras, and in certain geographical parallels, the investigations to which it has conducted are likely to have an important and permanent influence on geological observation and theory. Already, in the countries best examined,—in England, France, Germany, in Europe generally, and in North America, it is found possible to determine one or more periods when great and extensive subterranean pressures broke the submarine crust of the earth, and raised particular tracts of land above the reach of further marine deposits. Comparatively short periods of widely extended disturbance in the equilibrium of heat are thus clearly established, in alternation with far longer periods of repose in the same regions; and though it may be rather a coincident than a dependent phenomenon, it is not to be doubted that, among the older strata, these critical periods of disturbed equilibrium of heat correspond to critical periods in the revolutions of organic life. That either of these results is true universally would be a ridiculous affirmation, in the present state of our ignorance concerning immense areas of the globe; but it will not be the less useful to exemplify their truths, chiefly by application to the British islands. The following table is intended for this purpose, and may be compared with that on page 152., which contains some of the same elements:—

Primary period, of ordinary action, among the aqueous and igneous agencies; the ancient bed of the sea was filled with sediments, the most recent of which obviously were derived from tracts of land, which are now for the most part submerged. The organic remains of this whole period really compose but one series, in the same sense that the fossils of the oolitic or cretaceous system are one varied group. There were local disturbances of the sea bed in the Cumbrian and other districts.

An interval of dislocations followed, in which all the primary strata of England, in every part (excepting perhaps the silurian region), were raised to angular positions, so that the next system rests unconformedly upon them.

Carboniferous period, of ordinary action; the sea filled with new sediments by inundations from the land which had been lately and previously uplifted. The series of organic remains undergoes an entire and apparently sudden change of species.

Another interval of dislocations, so general and remarkable, that there is not a coal field in Europe which appears to have been exempt from them. The geological date is not always assignable, except within the limits of the uppermost coal deposits, and the base of the new red sandstone. The whole period of rotheliegende and magnesian limestones may be included in this interval, and some of the peculiarities of the saliferous system are probably the effects of this great disturbance.

The oolitic and cretaceous periods appear to have been scarcely broken by any violence in the region of the British isles, but the whole bed of the sea underwent a gradual and continual rise, which brought up progressively the north-western parts of the oolitic rocks. (On the continent of Europe the oolitic and cretaceous periods were divided by an interval of great disturbance.)

An interval of extensive dislocations has been recognised by M. de Beaumont and others, under the title of the Pyreneo-Apennine system; in England the effects of disturbance are chiefly exemplified in the conglomerates and pebbles which abound in the lower tertiary strata.

The eocene period of Mr. Lyell succeeds, with a prodigious number of organic forms, almost wholly distinct from those of the older strata.

The dislocations of the western and eastern Alps, combined with the evidence afforded by diluvial phenomena and raised sea breaches in many parts of the world, appear to show a separation between the eocene and modern periods by a period of violent disturbances, connected with the rising of some of the highest mountain ranges in the world. The conjecture of De Beaumont, that the elevation of the Andes was one of the latest of these great disturbances, has been verified by the researches of Mr. Charles Darwin in Patagonia and Chili. (Geol. Proceedings.)

Modern Period of Ordinary Action.

The value of such an arrangement as that here presented is not in its minute accuracy, but its general application; and in this respect it is, apparently, worthy of considerable confidence. It is however impossible to assert, or to believe, that the intervals of disturbance were very short, or that a mountain range rose in a moment, to divide an ocean and change the relations of organic life. The alternation of great periods of repose and disturbance, in every district yet examined, is certain; the correspondence of these periods in remote regions, though not completely proved, is rendered probable; and it only remains to see what is the bearing of this discussion upon geological theory.

Such alternations of repose and violence appear a necessary consequence of the gradual refrigeration of the globe; the duration of repose and the violence of the disturbance being dependent on the resistance to pressure offered by the consolidated crust of the earth. However hot a planet may have been, it is conceivable that in time sufficiently long the radiation of its heat into the cold ethereal spaces must continually reduce its internal temperature. The solidified crust, when cooled to the temperature derived from the joint influence of the hot sun and the cold regions around the globe, suffers no further loss of heat; but the internal parts may still grow cooler through immense periods of time; they may thus contract more than the outer parts, and fail to sustain them; fractures follow, and the equilibrium of pressure is restored, till a long period of cooling revives the irregularity of forces, and the crust breaks again. Periods of ordinary, and intervals of critical, action are direct consequences of the Leibnitzian doctrine.

This however does not prevent the favourers of the contrary hypothesis from adding to their speculation of the constancy of natural forces the further assumption that they are subject to a cycle of large variations, such as those "perturbations" which affect even the regular orbits of the planets. Such cycles of variation have been suggested, but unless a cause be assigned (as is done for the planetary perturbations), this gratuitous addition of one hypothesis to another weakens the probability of both. This appears to us an impartial view of the subject.

Climate.

That during early geological periods, the northern zones of the earth enjoyed a climate approaching to that which is now confined to the equatorial regions, is admitted among the established inferences of geology, upon the evidence of the remains of plants and animals found imbedded in the strata. For reasoning on this subject which we deem satisfactory, the reader may consult a former chapter of this work. (Vol. I. ch. v.) A true geological theory must be capable of fully accounting for the change of temperature which has thus affected large regions of the globe.

Besides the general speculation of a refrigerating globe, we have on this subject three others to examine. The hypothesis advanced by Mr. Lyell is founded on the acknowledged fact that the mean temperature of any point on the earth's surface is liable to considerable variation, according to the position of land and sea. By supposing a peculiar distribution of masses of land, equal in area and elevation to the present continents and islands, this eminent author endeavours to account for the facts regarding ancient climate, without calling in aid any external or internal sources of a change of heat.

There are, however, two external sources of change of the mean temperature of the whole globe. The calorific influence of the sun may increase or diminish, because the mean distance of the earth from that luminary is subject to variation: the temperature of the ethereal spaces in which suns and planets move may not be the same in every part; and, if the whole solar system has a movement of translation in space, it is possible that in some former period the earth may have passed through regions of the universe which communicated heat instead of abstracting it.

We shall first notice the speculations which relate to external sources of heat and cold.

The variability of solar heat, as bearing on geological problems, has been investigated by Sir John Herschel. It is known that the major axis of the earth's orbit is invariable, but that the minor axis is subject to considerable change in a long period of time, though the limits of the variation of eccentricity which this produces in the earth's orbit are unascertained. This eccentricity is at present, and has been for ages beyond the reach of history, on the decrease, because the minor axis of the earth's elliptic orbit is continually lengthening. The limit of this elongation is now nearly reached, for the orbit has become nearly circular.

It must be very obvious that the amount of solar heat received on the earth (the major axis of the orbit being constant) diminishes as the minor axis is elongated, and, therefore, the earth's heat derived from the sun has been through all historic time, and is at this moment, on the decrease. The quantity of solar heat received on the earth, is, in fact, inversely proportional to the length of the minor axis of the orbit; and were the limits of the variation of this axis calculated (which would be excessively laborious), the extreme change of climate from this cause might be known. Taking, however, the extreme measures of eccentricity, which occur in our planetary system (Juno and Pallas for example), as possible in the case of the earth, Sir J. Herschel deduces from calculation that the utmost difference of mean solar radiation might amount to about three per cent., a quantity certainly very small, and altogether inadequate, except by a peculiar combination of favourable circumstances, to account for the changes of climates established by geological observations. Until the calculation alluded to be actually made, it appears unreasonable to attach much weight to this source of variation in climate.^[6] The solar heat annually poured upon the earth is stated by Pouillet to be sufficient to melt a coat of ice 14 metres thick, encrusting the whole globe of the earth.

2. The heat of the planetary spaces is a subject on which, Mr. Whewell justly observes, the scientific world has hardly yet had time to form a sage and stable opinion. Fourier has asserted the existence of a definite temperature in these spaces, and ascribes it to the radiation of the fixed stars in every part of the universe. He assumed this temperature at about 50° centigrade below the freezing point, and Swanberg has been led, by a wholly different line of reasoning, to nearly the same result, as to the degree of temperature of the void spaces of our system.

This view of the state of the ethereal spaces is important in the application of the mathematical theory of heat to the present and former conditions of the earth. But M. Poisson, while fully admitting the existence of considerable heat below the surface of the earth, and the comparative cold of the spaces which now surround our globe, assigns the following reason for the high temperature below the surface. The cosmical regions in which the solar system moves have a proper temperature of their own, and this temperature may be different in different parts of the universe. The earth, in whatever part of these spaces it be placed, must be some time in acquiring the temperature of that region, and this temperature will be propagated gradually from the surface to the interior parts. Hence, if the solar system moves out of a hotter into a colder region of space, the part of the earth below the surface will exhibit traces of that higher temperature, which it had before acquired. Thus, without supposing great heat in the whole mass of the interior parts of the earth, the phenomenon of augmenting temperature below the surface would be explained.^[7]

Geologists will probably be pardoned for not attaching importance to this remarkable speculation, except for the proof it affords that men of enlarged conceptions, and the highest mathematical endowments, regard the facts already known by observation of the heat now present within, and the climate which anciently overspread the earth, as inexplicable, except by general variation of heat through a considerable part of the mass of the earth, or even a great range of the cosmical regions. Local sources of heat are deemed inadequate, and left unnoticed by Poisson, Fourier, Arago, and Herschel.

We have therefore finally to compare the account of the changes of ancient climate, proposed by the distinguished advocate of "modern causes," for comparison with that furnished by "refrigeration of the globe."

The principle of Mr. Lyell's hypothesis of changes of climate, in different geological periods, is the change of position of the land. We have already stated as a main cause of the differences of the mean annual heat at places which lie in the same zone of latitude, and consequently receive the same quantity of solar radiation, the influence of oceanic currents. The tides raised in the equatorial seas circulate round the globe, and, by spreading up the North Atlantic and North Pacific communicate warmth to the western shores of Europe and America. Oceanic currents, arising from other causes, mix the temperature of different latitudes, and moderate the extremes of heat and cold. Nor is this all. The higher that land is raised into the atmosphere the colder does it become, and the larger the mass of this elevated land the more powerful is its cooling influence on the vicinity. For this reason, the mean temperature of North America and Northern Asia is generally much lower than that of Europe in the same latitudes.

The nearly meridional band, which has the highest mean temperatures on given latitudes, passes up the Atlantic, along the west coast of Europe. In latitudes below 30°, the difference between the temperatures on this line of greatest heat, and those of America and Asia, though perhaps always sensible, is slight; but on arriving in high latitudes, the contrast is somewhat startling. Upsal, in latitude 60° N., has about the same mean temperature (42°) as Quebec, in lat. 47°. The isothermal line of 32° crosses the North Cape in lat. 70°, and from this vertex of curvature descends southward by the south side of Iceland, and the south part of Greenland, to the north point of Labrador, almost to lat. 60°. This is the most southerly part of the curve, which then bends to the north, and reaches 65° at Great Bear Lake, beyond which its course has not been completely traced. In the other direction, from the North Cape, this line deviates to the south, till it crosses the Lena below lat. 65°. Thus on the line of 32° it rises in the meridian of Norway 10° of lat. further north than in America, and 5° further north than in Asia. Nearly similar results follow from tracing the other isothermal lines determined by Humboldt in high northern latitudes, but the difference above stated is more than the average. In the same latitudes, Europe is warmer than North America by 5° Fahr. or more, but in particular situations this difference is much greater, amounting, in extreme cases, to 11°, or even to 17°. Such uncommon differences, however, are unimportant in a general argument.

Some portion of the great difference of the Atlantic and the continental climates may safely be ascribed to the gulf stream, which carries the warmth of Guinea even to Spitzbergen (according to Sooresby); but without this aid, a deep polar ocean communicating to equatorial seas must always mitigate the cold of the Arctic zone along the main channel of connexion, as a mass of Arctic land lowers the mean annual heat of the temperate zones, by collecting an eternal covering of snow and ice. On the contrary, in both respects, land in the equatorial regions may absorb more heat than water; and thus we have, as general conclusions, the greatest uniformity of climate, with the greatest expansion of sea; the greatest mean annual heat toward the poles, with equatorial land and polar oceans; and the least mean annual heat with polar land and broad equatorial sea.

If, therefore, during one long geological period, land of the same extent as that now above the waves, and rising to the same height, were situated round the poles, while the zones of the earth, which received most solar rays, were occupied by sea, there can be no doubt that the mean annual heat of the whole terraqueous surface might fall considerably, the greatest depression being in the polar regions. Such a state of things is fancifully called by Mr. Lyell the winter of the "great year". On the contrary, with continents equal to the present placed on the equator, and wide oceans overflowing either pole, there would be an augmentation of mean annual heat, and the "summer" of the great year would have returned!

Several questions, however, remain to be answered, before this elegant hypothesis can be embraced as a sufficient cause of the changes of climate, which appears to have come over the northern zones.

The collecting of land around the poles, or on the equatorial line, or in any other position, is not positively contradicted by known geological facts, but neither is any decided support given to the assumption by those facts: it cannot even be declared to be probable or improbable, on the ground of observations; for though these certainly teach us that the position of land and sea is indefinitely variable, they have determined little or nothing concerning their actual distribution in former geological periods.

This speculation is then purely hypothetical and framed to suit the phenomena, as others may be, and have been; but it involves no physical improbability on a great scale, and its details are based on real causes. We may, therefore, inquire farther, whether it is sufficient to explain the facts admitted concerning ancient climate. If we take the oceanic polyp aria, which abound in reefs among primary and carboniferous strata, as a mark of climate not inferior to that of the coolest regions where now coral reefs are formed, the mean temperature of the sea in the latitude of Christiania, situated on what is now the warmest band passing across the isothermal lines (now about 43°), must have been about 20° F. higher, which, added to the Already existing excess of temperature on this line above the average, makes nearly 30° F. for the necessary augmentation of marine temperature toward the north pole.

On the land, a very similar augmentation of temperature must be supposed: for the evidence of the arborescent ferns and fluviatile reptilia goes very much to establish the necessity of a mean temperature of above 60°, wherever the coal deposits spread in great abundance. Taking the coalfield of Edinburgh as an example, the mean temperature of the ancient land which supplied the plants there buried and changed to coal, may have been about 15° hotter than now occurs on this warm meridional band. It may, indeed, be supposed that these plants were drifted from southern lands; but what is the inference from observation? It is exactly the contrary, according to the evidence furnished in the Illustrations of the Geology of Yorkshire; where both for the oolitic and the earlier coal strata, it is proved that the drifting was from the north.

Surely these are serious obstacles to the reception of the hypothesis of change of ancient climate, by altered position of land and sea, on the ground of its being sufficient to meet the phenomena. In general, perhaps, we may venture to remark that it is unsafe to push the opinion of the possible average change of temperature in extra-tropical regions, beyond the extremes now observed therein. America, with little north tropical and wide north polar land, gives us a case of extreme refrigeration from the pole toward the equator; Africa and the west of Europe compose a surface of wide and hot north tropical land, with free channels ta a polar sea. The extreme difference of these extreme climates does not, we believe, in any two points of like elevation reach 20°, the half of which is, perhaps, more than the extreme excess or defect of heat beyond the average of the latitude at any one point upon the surface of the earth.

If an average excess of 10° of temperature be allowable according to this hypothesis, the extreme excesses may have been somewhat greater; but from the conditions of the hypothesis they cannot be taken to be so great as the extreme excesses now observed on the globe, but must be supposed comparatively small.

We have, therefore, only further to inquire in what manner the doctrine of progressive refrigeration of the globe from the earliest periods meets this case of the change of climate in regions far from the equator. Some geologists appear to have adopted, on the subject of the earth's interior heat, a singularly erroneous opinion viz. that a cold solid crust and an incandescent nucleus are incompatible. The doctrine of "central heat" (as the Leibnitzian speculation is sometimes inaccurately termed,) is, upon this false notion of the conduction of heat, declared to be a physical mistake. Yet it can be easily shown, both by experiment and mathematical calculations, to be a necessary truth) in a body circumstanced as the earth really is. If one end of a bar of metal, a few feet long, be plunged in the fire, while the other end is wrapped in a wet cloth, the one end may be ignited to any desired degree, while the other can be kept at any required temperature above a certain point, depending on the heating and cooling powers applied to the ends of the bar, its length, and the conducting and radiating powers of the metal. Instead of the metal bar, submit to the same heat a bar of stone, or a rod of glass; in these cases, unless the bar be very short, no cooling power at all is needed further than that of conduction and radiation from the surface of the bar, because of the extreme feebleness with which heat passes through its interior parts. What is the difficulty of applying this reasoning to the stony crust of the earth?

Fourier has done this in a manner which mathematicians deem admirable and satisfactory, in his masterly ‘Treatise on Heat’, now become the standard book of reference in the highest department of this subject. We shall use the words of one who has examined the arguments of Fourier.^[8] "Some of the results of this theory are fitted to make less formidable the idea of having a vast abyss of incandescent matter within the comparatively thin crust of earth on which man and his works are supported. It results from Fourier's analysis, that at 20,000, or 30,000 metres deep (12 to 18 miles) the earth may be actually incandescent, and yet that the effect of this fervid mass upon the temperature at the surface may be a scarcely perceptible fraction of a degree. The slowness with which any heating or cooling effect would take place through a solid crust is much greater than might be supposed. If the earth below 12 leagues depth were replaced by a globe of a temperature 500 times greater than that of boiling water, 200,000 years would be required to increase the temperature of the surface 1°. A much smaller depth would make the effect on the superficial temperature insensible for 2000 years. It is calculated, moreover, that from the rate of increase of temperature in descending, the quantity of central heat which escapes in a century through a square metre of the earth's surface would melt a column of ice having this metre for a base and 3 metres for its height."

Now it follows as a necessary consequence of the progressive refrigeration of the globe, that whatever be at this time the influence of interior heat upon the temperature of the surface, it was in early geological periods far greater than at present, and has been slowly diminishing, till, in Leibnitz's words, a consistent state of things is reached^[9]: for both theory and observation agree in showing that internal heat is almost insensible among the other elements of climate. During the last 2000 years it is calculated that the cooling of the globe has not lowered its surface temperature $\scriptstyle {\frac {1}{300}}$th of a centigrade degree; for had this been the case some change in the length of the day would have become perceptible since the era of Hipparchus. This fact has sometimes been urged as an objection to Fourier's conclusions, though it is really a corollary from the theory, and its agreement with observation might have been, with equal justice, mentioned in corroboration of its truth.

It is very conceivable that, in the earlier stages of the cooling of the globe, a moderate general warmth of 30°, 20°, 10°, &c. might be successively communicated from the interior to the surface; and it has been already seen that this uniform addition to the effects of the solar radiation would supply in northern zones as far as 70°, 60°, 50°, 40°, &c. of latitude successively, the temperature requisite to allow of coral reefs in the sea, palms and tree ferns upon the land, and crocodiles and other huge reptiles in the rivers and estuaries.

On the whole, until the sufficiency of a peculiar position of land and sea, to meet the phenomena of a change of climate is proved, and some independent ground of definite probability is assigned for the occurrence of such a position, it would be premature to recognise in the present aspect of the hypothesis which proceeds upon that assumption the features of a true theory. But it would be equally unjust to condemn it as false, for it is not disproved, and no one has shown that such positions of land and sea as Mr. Lyell has contemplated, may not acquire a determinate probability among other consequences of a general theory. In the mean time that admirable writer has conferred no small benefit on geological theory by introducing for consideration this elegant and consistent speculation.

Conclusion.

That the doctrine of progressive cooling of the globe is to be now received as an established theory, those who desire the real progress of geology will prevent themselves from affirming; and perhaps few who have attended to the inferences contained in these volumes will hesitate to believe that it will one day become so. It is no small argument in favour of this hypothesis (as it must still be called), that it appears to include, easily and obviously, so many of the leading and general truths established by geological observation. The figure of the earth, its density, the actual temperature of its surface and interior parts; the general floor of igneous rocks below the strata; the repeated formation and uplifting of such rocks; the great and systematic fractures of the earth's crust; are all capable of explanation by this one consideration. Moreover, it assigns a reason for the remarkable uniformity and extent of the earliest as compared with the latest deposits of water; and accounts for the characteristic induration of the ancient rocks, the rarity and even total absence of organic remains in them, the changes of climate, and the periods of ordinary and critical action, which observation has established, by one and the same principle. The proximity of heated masses to the surface in the early ages of the world, to which these phenomena are easily referred, is indeed hardly doubtful, since it is equally indicated by a full investigation of the sources and distribution of terrestrial heat at this day.

What then is wanted to turn this apparently fortunate speculation into an established general theory? It is the same process which has given stability to the idea of gravitation, and is now employed to sustain the undulatory theory of light. It is the deduction of characteristic phenomena in the real order of their succession.

To this task geologists, as such, are quite unequal. The preliminary investigations in mechanical and chemical philosophy are yet incomplete; we do not know to what extent the earth, in its interior parts, is solid or liquid; we cannot affirm in what state of combination the substances there occur; the rate of increase of heat below the surface is only approximately determined in particular regions; the depths of the sea have not been measured; the geological surveyor has not visited above half the globe; the true relations of the existing creation of life to those which have passed away are yet the subjects of discussion; the times which have elapsed during the accomplishment of geological revolutions a;e not even reduced to conjecture!

Yet in spite of these disadvantages, the conviction is spreading that some good will result from even an unsuccessful attempt to deduce mathematically the main consequences of the Leibnitzian speculation. To this task Mr. Conybeare invited attention in 1831; and since that time Mr. Hopkins has given proof, in more than one Memoir, that the subject is in able hands. The mist is gradually disappearing; and if we see not clearly the high point of truth which we desire to reach, and which may yet be far distant, at least the direction of our march is found; and though the paths may be devious and hazardous, they are full of beauty and delight.

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1. Sir John Herschel, in his Discourse on the Study of Natural Philosophy, p. 287.
2. Gardner, in Geol. Proceedings, 1853.
3. Phil. Transactions, 1833.
4. See p. 241. On the subject of the Chilian earthquake, consult, generally, the Geological Society Proceedings, vols. i. and ii.
5. In Ann. des Sciences Naturelles for 1829-30.
6. See Geol. Trans., 2nd series, vol. iii.; and GeoL Proceedings, vol. i. p. 245.
7. See Mr. Whewell's Report and Communications to the British Association, 1835.
8. Whewell's Report to British Association, 1835.
9. "Donee quiescentibus causis, atque æquilibratis, consistentior emergeret reruin status." (See Conybeare, Report on Geology, to British Association, 1832.)