A Treatise on Geology/Chapter 1
A
TREATISE ON GEOLOGY.
1. Objects of Geological Science..
THE phenomena of geology are so various and complicated,
that hardly any class of writers has left them
wholly untouched; and the aspect of this science changes
according to the peculiar object of different inquirers.
Strabo, accustomed to enlarged views of physical geography,
spoke of the existing forms of the surface of a
part of Asia Minor, in reference to the ancient revolutions
of nature which had occasioned them, and thus
appeared to include geology among the tributaries to
physical geography. Werner, habituated to minute
discrimination of minerals and rocks, regarded the
science, which sought for philosophical explanations of
these differences, as a branch of mineralogy; while in
Hutton's comprehensive mind geology applied itself to all the variable conditions of our planet, in all ascertainable
or conceivable times, past, present, and to
come.
Nor is there less diversity of theme among the zealous students of geology. For some regard, as the principal object of their inquiries, the constitution of rocks and minerals,─the chemical effects which are traceable in the earth, and thus merely enlarge the limits of mineralogy; others turn their whole energy to the development of the history of fossil plants and animals, and thus constitute the interesting branch of organic geology[1]; while a third class watches the relation of the phenomena established by geologists to general statical and dynamical truths, and thus strives to found the new and attractive study of physical geology. Nor is there in this apparently imperfect apprehension of the extensive range of geological inquiry much to be reprehended. It is desirable that as many roads as possible should be opened toward the attainment of the varied truths which must be collected, by insulated observers, before it can be practicable for even the most comprehensive intellect to frame a general and consistent view of the whole scope of this great branch of the study of nature.
The natural sciences are commonly understood to include all inquiries into the history of the inorganic and vital phenomena which take place on and about the surface of the globe, and the relation of these to general terrestrial, and cosmical conditions. The problems thus proposed for discussion are sufficiently ample and diversified to employ a very large number of observers and reasoners; they are, perhaps, inexhaustible; yet, were they all resolved, the study of nature would not be ended; for there would remain the inquiry, whether the present condition of nature is to be taken as a type of all her past and future states, or to be viewed as exhibiting one of many aspects, one of many gradations of change—a temporary adjustment, not a continued equilibrium—the last known term of a series whose law of variation is to be discovered.
Taking up this idea of the aspects of nature, at any particular epoch of the history of the earth (since the present laws of terrestrial and cosmical phenomena were appointed), being the last term of a long series—the complex effect of many anterior influences—we may proceed to inquire what monuments remain in nature of any previous aspects or conditions, and from these to infer the nature and amount of the agencies formerly concerned in producing and varying them. Having ascertained the series of past changes, we may venture to speculate on the future revolutions of the face of nature, to which the law of variation of the agencies concerned must necessarily lead.
Now, these are precisely the problems which it is the province of geology to consider. Gathering, from the labours of mineralogists, botanists, and zoologists, a knowledge of the existing species of inorganic and organic bodies; from geography, the account of the present configuration of the surface of the globe; from general physical researches, what is known of the constitution of the atmosphere, the ocean, and interior of the globe,—geology proceeds to inquire further, whether the mechanical, chemical, and vital phenomena, formerly exhibited on the earth, can be traced in their effects, so as to be put in comparison with those daily occurring; whether the mineral products of earlier eras of the world were identical in kind, and equal in rate of production, with the modern products of this description; whether the plants and animals of the ancient world were of the same structure as those which now adorn its surface; and whether the general physical conditions to which all these are correlated have always exerted the same kind and degree of influence as at present. In whatever way these questions are answered, they inevitably lead to speculation as to future revolutions which may vary the face of the globe; and thus geology, far from being the limited and narrow subject which was illustrated by Werner, after the model of the Saxon mountains, is found to include, perhaps, the largest class of inquiries which has ever been ranked under one head. For, in the attempt which it makes to decipher the history of the past, and to prognosticate the future changes of our planet, it requires the aid of all the gathered knowledge of nature, interpreted by the profoundest researches of abstract science. It is not even enough to know the actual state of the earth, we must further learn the measure of momentary changes in this state; and thus inquiries of a new order are suggested to naturalists, who are seldom aware, while investigating the problems before them, that these include dynamical as well as statical determinations, and that the former are necessary to the right understanding of the latter.
It has been made a reproach to geology, that, in its bold attempts to penetrate the dark veil which time has thrown over the mysteries of ancient nature, it has wandered far from its just mark, beyond the boundary of inductive philosophy and mathematical laws, into the unprofitable regions of cosmogony. Geologists have been equally blamed for stopping short of attainable truth, and declaring, upon inadequate grounds, that the earth shows no trace of a beginning, no prospect of an end. But geology is really distinct from cosmogony. Inquiries into the origin of the planetary bodies belong to one of the highest of human sciences,—astronomy. Geology, far from intruding within its precincts, supposes the globe to be constituted as a planet moving round the sun; takes for granted all the laws which this relation implies, and limits itself to the discovery of the strictly terrestrial phenomena which have happened upon this globe under these conditions. On the other hand, it was rash and presumptuous to assert the invariable uniformity of natural causes and effects, through all past time, upon data so insufficient as those known to Dr. Hutton; and, even if this were established, it would not be sound logic to infer that the earth showed "no traces of a beginning no prospect of an end"; because, on that matter, other evidence might have been brought to bear.
But yet a candid reasoner need not greatly reprehend either the wandering of geology into cosmogony, or the too forcible acknowledgment of the incompleteness of all human research into the origin of the visible system of nature, which is contained in the much-censured language of Hutton. Neither of these errors is likely to be very injurious in science; nor can either be justly charged with the slightest tendency to make men forget that all the arrangements of nature are but the expression of the will of the Supreme Creator and Lawgiver of the universe. Let us not, therefore, be checked in our inquiries into the history of the globe, by anything but the good rules of philosophising, which are essential to the right use of the intellectual strength which God has conferred upon man, to be exercised on the mighty works of nature; and least of all let us be deterred from the pursuit of truth, by the vain and impious dread that we may go too far, and penetrate too deeply into those mysteries which, among their other uses, have this one,—that they continually excite to activity the soul of man; and, the more they are studied, lead to deeper delight and more awful contemplation of their glorious and beneficent Author.
Geology, or the natural history of the earth, as a planet revolving in space round a central orb of light and heat, surrounded by an atmosphere, and partially or wholly begirt by water, includes all the phenomena produced on the land, in the sea, and within the mass of the globe, by the operation of those mechanical, chemical, electrical, and vital forces, which are termed natural agencies, from the earliest epoch of which monuments remain, down to the present hour. All variations in the proportions of land and water; all variations of level; all changes in the combination and distribution of masses or molecules of matter, whether above, upon, or below the surface; all changes in the number, structure, forms, and relations of plants and animals, are to be ascertained as facts, and employed in reasoning, for tracing a general and continuous history of the physical revolutions which the earth has experienced since it became a planet.[2]
In this comprehensive definition of the objects of geological science, we include its real and legitimate field of research, without interfering in the least with the independent exercise of collateral sciences. To astronomy belongs the investigation of the history of the globe, as a part of the planetary system, and the results thus reached help to correct and limit geological inferences. Chemistry employs itself upon the inquiry into the laws and modes of mutual action among the particles of matter, and gives its results to aid in the general history of terrestrial phenomena. It is the province of zoology and botany to arrange and interpret the facts connected with life and organisation in plants and animals; and to these branches of knowledge geology owes immeasurable obligation. Thus the study of the ancient natural history of the earth draws help from every kind of inquiry which man can make into the actual constitution of nature, but robs none of its interest or glory; on the contrary, by the novelty of its discovered facts, fresh problems are presented to the cultivators of natural science, and a perpetual excitement is kept up, which has proved of infinite service to them all.
2. Means of Geological Investigation.
THUS ample and various are the problems clearly proposed
in geology: in order to see how far they are determinate, and capable of solution, let us survey the
means of investigation placed within our power, from
what sources the data are to be collected, and in what
manner the methods of interpretation are to be discovered.
In existing nature, we can examine and collate, for purposes of reasoning, three orders of data; viz. facts relating to organic beings; facts relating to inorganic bodies; facts concerning physical conditions, or modes of dependence of these two classes on each other, on the general laws of matter and motion, and the arrangements of the solar system. In this manner, it is seen that the radiation of light and heat from the sun; the alternation of day and night, caused by the rotation of the earth on its axis; the variations of seasons, depending on the position of its axis and on its revolution round the centre of the planetary system; the existence of an ambient atmosphere; the inequality of elevation of land; the distribution of land and sea, and other cosmical and terrestrial conditions, have a distinct influence on all the arrangements of organic life, and on some of the phenomena of inorganic bodies. Sometimes the law of this dependence is evident; generally the nature of it is discoverable, always the fact is capable of being satisfactorily ascertained: and thus existing nature is presented to our minds as a system of beautifully adjusted parts, which it is the highest province of the noblest intellect to contemplate in one point of view, and pourtray under the aspect of a general theory.
In geology, however, the physical conditions are not known, but are to be inferred, for any particular epoch in the history of the globe, from the facts collected concerning the organic and inorganic bodies which belonged to that era, and are indicative of the physical influences then operating: and hence arise both the difficulties and the charms of geological reasoning. The difficulties are unavoidably very great; for, even in the acquisition of the data concerning organic and inorganic bodies, geology is often forced to be satisfied with less exactness than is attainable in modern natural history. Certain shells, fishes, &c., are recognised by the geologist, but he may not find them of all ages; cannot know correctly what were the limits of variation, of their magnitude, forms, habits, &c. In some important cases, he can hardly know whether particular mollusca and fishes lived in lakes, rivers, or the sea; whether certain plants grew on land or in water. Now, as it is only on a correct knowledge of the affinities which the fossil remains of life bear to existing races, that any just inference can be founded concerning the contemporaneous physical conditions, we see how fertile are the sources of error, and with what justice men of philosophic minds have endeavoured to restrain that propensity to speculation which imperceptibly gets possession of the human mind, and has particularly luxuriated among the enthusiastic votaries of geology. There is less difficulty with respect to inorganic bodies, because the laws of their aggregation are such as, in many instances, can be tested by experiments which apply with equal exactness to the ancient and modern mineral kingdoms.
Scale of Geological Time.
It is further to be observed, that the very foundation of all history of geological phenomena is difficult to fix; for if it be embarrassing, even in civil history, to ascertain the relative dates of many most important occurrences, how careful should we be in marking even the order of succession of geological phenomena of the same kind; how diffident of our power of determining at all the lines of contemporaneity among occurrences of different nature, which happened in different regions of the globe, and under at least some difference of physical conditions! The very first inquiry to be answered, then, is, what are the limits within which it is possible to determine the relative dates of geological phenomena? For if no scale of geological time be known, the problem of the history of the successive conditions of the globe becomes almost desperate.
There is, however, at any place considered alone, a scale left us in the crust of the earth, by which to measure exactly the order of antiquity among the terms of the series of organic life, and to compare the relative antiquity of these terms at different and remote places, often with perfect satisfaction, and generally within moderate limits of error. This scale is the series of stratified rocks; and thus a great difficulty is overcome, and many of the inorganic and organic productions of older nature are capable of being arranged in the order of their successive appearance. We must, therefore, explain the nature of this fundamental scale, and illustrate its application; for we are, perhaps, not in a state to define the extent of its applicability.
Series of Stratified Rocks.
The crust of the earth is, for the most part, stratified—that
is to say. the most abundant of the materials
whereof it is composed, are in the form of widely extended
and comparatively thin layers (called strata),
laid one upon another, to a great numerical amount;
these strata were, beyond all question, deposited in
water, because many of them contain marine or fresh
water shells, fishes, corals, and other marine exuviæ
(and even were this not the case, the fact of the production
of analogous or very similar strata beneath
modern waters would justify the inference); therefore,
the lowest were formed first, the uppermost last. To
attempt proof of such a proposition would be to outrage
common sense: he who cannot supply to himself
the proof that the lowest layers of sediment produced
by the waters of a pond, lake, river, or ocean, were
deposited before the upper ones, is incapable of apprehending
any natural truth. Yet, upon this simple
and self-evident proposition rests the whole body of
geological inferences which include relative time.
That the lower strata are the oldest, the uppermost the most recent, is a truth independent of all circumstances but the fact of the rocks being stratified and of aqueous origin: but what is the interval of time? Whether it was long or short—a day or a thousand years—how much older one rock is than another—if ascertainable at all, cannot be known without adding to the fact of the order of succession a number of other circumstances, characteristic not merely of succession, but of duration.
Lapse of Time inferred from the nature of the Series of Rocks.
The circumstances which help to define our notions of the time elapsed in the formation of the crust of the globe,—to translate, as it were, the symbolical notation of the geological scale of time into intelligible periods, having relation to the duration of the human race,—are various, and all concur in impressing the mind of a candid reasoner with ever growing convictions of the immense antiquity of the globe; the many long periods of geological changes which it has experienced before arriving at the state when, in the magnificent language of holy writ, it was said to be expressly re-arranged for the creation of man, and the present system of terraqueous conditions.
The historic records of man's residence on the earth are, for most parts of the globe, utterly incomplete; so that, but for the Jewish Scriptures and other documents of eastern nations, we should be in danger of attributing to the human race an origin too recent by thousands of years. Now, as all historic records end, for each country, with the surface,—terminate at some point of man's history posterior to the preparation of that tract for his residence, we see how far more ancient than the historic date of the human race is the series of productions which lie below the surface. The limit of least antiquity of the scale of geological time is in every country beyond the date of the present surface. The series of strata is so ancient, that even its uppermost and newest term in every country is older than the race of man now existing there; though we are not entitled to say, without further examination, that it is older than the human species generally, for it is supposable that a former race of men might have existed over an older surface of the same part of the spherical area, under older physical conditions.
Antiquity of the Surface.
The records and traditions of mankind, which give a few thousand years to the existence of the human species on the globe under its present physical conditions, are in some respects corroborated by geological evidence of the comparatively recent date at which atmospheric agencies and drainage waters began to waste the surface of the earth, under the present relations of the level of land and sea. The notices of Herodotus concerning the formation of the alluvial land in Egypt from the inundations of the Nile, and similar facts connected with other great rivers, combine with the elaborate arguments of De Luc, concerning the formation of deltas in the upper ends of lakes, instances of which abound in every country, to show that the actual relations of the level of land and sea are, for the most part, not of so ancient a date as to be beyond comparison with the traditionary dates of the antiquity of the human race.
Having thus adopted as the limit of least antiquity
of the scale of stratified rocks, the traditionary age of
the human race, let us turn to consider the nature and
meaning of the scale itself, so as to learn its value and
range in the interpretation of the phenomena which
happened in earlier physical conditions of the globe.
Nature of the Scale of Time.
The rocks composing the crust of the globe are for the most part stratified; but exceptions occur, especially in mountainous countries: the series of strata is commonly definite, or composed of a certain number of simple terms, i.e., layers, each of a particular quality, in every small district; considered with reference to very large districts, it is found that, by grouping together the layers in natural assemblages, the series of these compound terms is also definite: finally, on comparing the series of even remote tracts, the compound terms themselves combine into groups, which are ranged in the same definite order whenever present together; for in some countries whole groups are absent, and others interpolated in the series. It is clear, therefore, that amidst all the causes of local diversity in the series of strata, some general influences have prevailed to give a determinate analogy of character to the resulting succession of stratified rocks in all parts of the globe If we can search out the causes of local diversity and general agreement, and thus ascertain the law of the geological scale of time, nothing will remain to be done but the comparison of this law with the analogous operations of modern nature, in order to attain the most precise account of geological time which the human mind can reach.
Terms of the Scale of Geological Time.
The different strata which are terms of the series or spaces on the scale of geological time are of various mineral qualities—arenaceous, argillaceous, calcareous or composed of mixtures of these in unequal proportions. In the substance of many of them, peculiar minerals, as mica, red oxide of iron, silicate of iron, &c., are diffused; they differ in hardness, granulation, crystalline structure, and many other circumstances. Every one of these differences had its cause in some peculiar contemporaneous physical condition; these strata succeed one another in a settled order over the same area; were deposited beneath the water on the same part of the bed of the sea; it is certain, therefore, that in and about the same regions of the globe the physical conditions varied thousands of times during the formation of the series of strata. The mere inspection of one stratified rock composed of several analogous beds, gives a strong impression of elapsed time; but when we see thousands of beds of different qualities, the mind is opened to the further evidence which geologists bring on this important subject.[3]
Many, indeed most, of these strata contain the remains of animals which were living in the water at or before the time of the deposition of the rocks, and several are full of plants which were swept down from the dry land on which they grew into the ancient ocean, and then entombed in the strata at that epoch in progress of formation. By methods of undoubted accuracy, the length of life of some of these buried trees is ascertained to have been considerable that they lived a hundred years for instance; the shells entombed often show the growth from young to old during the formation of one or a few thin layers of rock. Thus, in many instances, we are forced to suppose the lapse of a period of years during the accumulation of even one thin bed of stone. And even if this conclusion were not circumstantially exact, if the shells of all ages, living together in the sea, were buried in one bed by one action, or even brought from small distances to be so buried, yet the inference is little altered by these admissions; for still, between the formation of certain beds, above and below those shells, their lifetime must have passed.
Series of Terms on the Scale of Geological Time.
But these conclusions become at once strengthened and more definite when we take into consideration the nature of the series of these terms; each of which indicates the lapse of time. For, first, it is found that the terms are recurrent, so that again and again similar or analogous strata are repeated, in different combinations, proving that the physical conditions which governed these depositions of strata were in some respects of periodical occurrence, or rather subject to interruption and fluctuation, so that different combinations prevailed at different periods. If we ask, in modern nature, so uniform in the local results of the same kind, the explanation of this, the reply will be immediately found: those periods of new combinations among the physical conditions of a given region are far beyond the range of human experience.
Moreover, an additional fact of great interest here comes to fortify all our inferences—the organic remains of plants and animals which abound in the earth are not those of the tribes that now live, but of many wholly extinct, and often quite different, races—different in form and structure, and, consequently, in function and habits of life, though certainly belonging to a general system of nature founded on analogous principal conditions. Further, it is not sufficient nor correct to say, there is one living and one extinct creation: the plants and animals buried in the earth belong to many distinct and successive creations, which differ among one another no less than they almost all differ from the actual forms of life. These distinct creations of former date are found buried in different parts of the series of strata; one series of organic forms belongs to the lower and older strata, another to those of middle, another to those of later date. The different groups of strata, deposited in successive periods, are thus filled with distinct races of plants and animals, which lived at successive periods, and thus it is proved that in every region the land and the sea were covered and filled at successive times with new creations suited to the new physical conditions of the altered planet.
This is not speculation, it is certainty. Each system or group of stratified rocks contains the remains of the plants and animals which existed at or previous to its production in or near the water in which it was formed: it is the museum of the period, the only repository of the monuments of that age of the world. By collecting these, and viewing them in the order of succession in which they occur in nature, we contemplate the forms of life which have successively occupied the globe, and by comparing them, on philosophical grounds, with the creatures that now exist, we can frame conjectures more or less satisfactory as to the state of the atmosphere, light, heat, and other circumstances, to which their life was adapted.
If we are to reason at all concerning the phenomena of nature, one of two conclusions must be adopted with reference to this subject; either the physical conditions whereto the existence of those plants and animals was related, changed gradually and equally in obedience to some continuous—law the forms of life being varied accordingly—or were liable to violent interruptions or revolutions, consequent upon new circumstances, or the accumulated tension of some feeble but continuous disturbing agency. Which of these views is true, will be the subject of inquiry hereafter: for the argument as to the lapse of geological time, it is immaterial which may be preferred; since in existing nature the rate of such physical changes, supposing them to be continual, is so small, as to have caused almost no changes of organic life in several thousand years;—witness the sculptured monuments of Egyptian grandeur;—and violent revolutions, capable of so influencing organic life (if probable at all), require, according to what is known of the earth and planetary system, periods or intervals too great for the mind to comprehend.
Interruptions of the Series of Time.
In certain rocks we find angular fragments, or rolled pebbles, derived by mechanical action from pre-existent and pre-consolidated rocks. The Righi, in Switzerland, is composed of such conglomerate masses;—the red sandstone of Cumberland and Westmorland is full of pieces of the subjacent slaty rocks;—the sands near London are stored with rolled flints from the subjacent chalk. The fragmentary masses, thus imbedded, are often the repositories of organic remains, sometimes of portions of mineral veins, both of anterior date to the rocks now including them. Thus we see proof of the occurrence of different modes of action over the same geographical areas, and our belief in the length of time requisite for all these occurrences becomes immovable.
In general the stratified masses of the earth's crust are placed with their surfaces parallel to each other; from which we know that during their accumulation no violent disturbance of the external parts of our planet happened in those regions to confuse the regularity, and alter the horizontal plane of deposition. But in particular instances this conformity of the strata is departed from, and certain (older) rocks appear inclined at various, often steep, angles, or standing even vertical, while the more recent deposits lie level or nearly so, upon them. What renders this case of disturbed stratification more impressive, with respect to the lapse of time, is the occurrence of positive circumstantial proof of the intervention of mechanical, chemical, or vital agencies of considerable duration between the elevation of the older, and the deposition of the newer strata, Thus in diag. No. 1. the inclined beds a of limestone were exposed to watery action, and broken up in part, so that fragments and pebbles of
them are found collected into beds among the mass of
later level sandy deposits b (Mendip Hills): and in
fig. 2. the same inclined limestone beds a are covered by horizontal oolitic strata c; and are worn and polished on the face of junction, and penetrated into holes by boring shells, which lived in the oolitiferous sea, long after the elevation of the older rocks. (1830.)
Length of the Scale of Time.
The scale of geological time given by the series of stratified rocks is one of unequal parts: for it is almost certain that the deposition of a given thickness of sandstone, was accomplished in a different time from that consumed in the production of an equal thickness of clay, limestone, coal, &c. Yet as many of the groups of strata contain both, sandy, argillaceous, and calcareous members, there is less error in estimating the relative periods which elapsed in the production of such groups by their proportionate thickness, than in applying the method to the several strata and beds of the groups.
In a general sense, then, the total length of the scale of strata is of importance, as an element for direct computation of the total time elapsed in the formation of the crust of the globe. This length in some cases amounts to more than ten miles, and is seldom to be estimated at less than five.
Means of Investigation of Facts.
Having now sufficiently explained the nature and origin of that standard of time to which all geological phenomena are to be referred, it remains to be seen, 1st. What are the means in our power for collecting the facts concerning chemical, mechanical, and vital phenomena, effected in ancient periods, which are to be combined into a history of the physical changes of the globe? 2d. What are the methods of interpretation applicable to these phenomena?
Direct observations of the mineral composition of the globe are the groundwork of geology; but were our knowledge limited to the depth which is reached by actual penetration of the crust of the globe, by pits, wells, and other excavations, or seen in isolated mountain slopes, it would be of little value for the object proposed.—The deepest mine in the world (Kitzpuhl in the Tyrol) is only 2764 feet below the surface, the loftiest summits of the Himalaya only ascend 28.000 feet above it.—Yet in consequence of the manner in which the stratified materials are arranged in the crust of the earth, it is possible, by proper combination of direct observations, to know the structure of the globe to the depth of three, five, or ten miles, according to the situation and circumstances of the country. This will be understood by attention to the annexed diagrams where fig. 3 represents a false, and fig. 4 a true, representation of the arrangement of strata, in a part of the crust of the earth.
If the globe were conceived to be cut through, the section near the surface would show a number of layers variously inclined to the horizon as in fig. 4., so as to come up to the surface in succession a, b, c, d, &c.; not, as in fig. 3., parallel to the horizon, as many persons are apt to imagine. The thicknesses of a, b, c, d, &c. separately, may be easily known by pits and wells, or natural sections in ravines or precipices; their order of succession may be found by the same means, and thus the total thicknesses of all the stratified rocks visible in any one country, may be easily known by direct observation. By a judicious selection of examples, the upper part of the series of strata may be measured in one district, as a, b, c, d, fig. 4, the middle in another, which contains the lower portions of the former series, as d, e, f, g, in fig. 4.; and the inferior portions in a third, fourth, &c., so as to complete one general table or section of the whole series of strata visible in an island, or continent; and, finally, on the face of the whole globe.
This labour is actually accomplished for many large portions of the globe; and it is found that the stratification of the matter of the earth ceases at some depth which is not the same at different places, three, five, ten, or more miles below which are rocks of different structure, aspect, and origin, and not stratified.
Here, then, is the limit of our knowledge, from actual inspection and exact induction of facts concerning the constitution of the earth: geology, as such, can penetrate no farther than this small fraction of the radius of the earth. But the far-searching power of mathematical science is capable, by correct interpretation of astronomical observations, and refined experiments on the specific density of the globe, of giving us some further information as to the nature and arrangement of even the central masses of our planet.
Direct observation of organic remains is the only source of information concerning the ancient orders of living beings, which were in existence at or previous to the deposition of the several strata: no reasoning a priori can be, in this inquiry, of the smallest service; but may be exceedingly injurious by infusing error and prejudices. It would be a gross error, for instance, to assume that the earlier forms of life were less complex in visible structure than those which now exist that the lower orders only of animals and plants had been called into being; for since the forms of life are most certainly made dependent on physical conditions, unless these latter can be known beforehand, there can be no reasoning on the matter, and there ought to be no speculation in inductive geology.
Means of Interpretation of Phenomena.
Admitting that by direct observation and the aid of higher science, geology has collected the evidence of the nature and arrangement of the mineral masses and organic reliquiæ, we may proceed to point out the method of interpretation which must be applied to the phenomena, in order to discover the physical conditions which prevailed in the several successive periods of the earth's structure in the situations observed. From the known to the unknown, through some common relations, has ever been the march of philosophical discovery: the skill of the general reasoner consists in the selection and use of these common relations for the determination of the principal conditions or agencies. It is through the knowledge of the conditions or agencies concerned in the phenomena of existing nature, that we must approach with caution to the solution of the similar problems offered by the phenomena of ancient times: the common relations are found by comparison of the analogous effects; but if the modern effects are merely known as laws of phenomena, and not reduced, to use Mr. Whewell's expressive language, to laws of causation, the corresponding phenomena of geology must remain equally unexplained.
The intelligent reader will easily see that it is not meant to convey the impression that nothing in older geology can be understood, unless there be known something exactly like it in modern nature; the laws of causation which regulate the phenomena now occurring on the globe, once correctly known, will certainly be recognised in a vast variety of older effects, in which the same agencies—however differently combined,—produced, or predominated so as to characterise, the result.
Thus the laws of chemical phenomena explain the production of the most ancient minerals, as well as of those daily produced before our eyes—the laws of physiology apply as well to the fossil flora, and the world of extinct animal life, as to the botanical and zoological enrichment of the actual land and sea: so also the laws of aggregation of sedimentary substances in water—of fused rocks and earthy matters the laws of optical and calorific phenomena—these laws of action are limited in their application only by the circumstances of the case or of the experiment, independent of time, and exempt from change.
Geology thus presents itself in an aspect which may surprise those who have not studied the philosophy of the subject. Though it gathers the most striking and beautiful facts, it depends for their interpretation entirely upon the progress of collateral science, and puts forth no speculation or hypothesis, except in conformity with the known laws of nature, and as a means of exciting and directing inquiry. In proportion as the philosophy of chemical, mechanical, and vital phenomena advances, so the interpretation of geological phenomena expands; and if at any time the leaders of geology have substituted conjecture for induction—a dogma for a dictum—they were then offending not so much geology as chemistry, physiology, and astronomy; and by these have they been justly condemned. Would that this warning might suffice to keep many hands to the sketch book and hammer, which they know well to use; and prevent them from attempting, without adequate precaution, to hasten the progress of geological theory.
- ↑ For this branch of Geology the term Palæontology is becoming usual.
- ↑ "Geology is principally distinguished from Natural History, inasmuch as the latter is limited to the description and classification of the phenomena presented by our globe in the three kingdoms of nature, whilst it is the business of the former to connect these phenomena with their causes." "It consists in the knowledge of the causes which have acted, and still act, upon this earth, and thus embraces all the knowledge we can gain of nature, by an attentive study of terrestrial phenomena."—De Luc, Lettre Premiere.
- ↑ The entire mass of our continents is composed of strata, similar in this respect to the regular courses of stones in our buildings. A succession of strata indicates a succession of time for their formation; and the change from one species of stratum to another placed upon it, indicates a change of cause. Thus is the mass of our continents the product of successive operations, during which the producing causes have undergone successive changes. We see, moreover, that many of these strata contain the remains of animals; and that in some successive strata these organized bodies are of different species. By this we judge that some considerable length of time was necessary for the formation of these strata, both on account of the succession of individuals of the same species of animals in some of them, and also on account of the change of species in the same places where the former are buried.—De Luc's Letters.