Popular Science Monthly/Volume 44/February 1894/The Position of Geology
| THE POSITION OF GEOLOGY. |
By Prof. JOSEPH PRESTWICH, F. R. S.
THE position of geology in this country at the present time, more especially as relates to the later geological periods, is anomalous and possibly without precedent. On one side its advance is barred by the doctrine of uniformity, and on the other side by the teaching of physicists. The former requires that everything should be regulated by a martinet measure of time and change. It asserts that the vast changes on the earth's surface, effected during long geological periods, are to be measured by the rate at which similar hut minor changes are effected in the present day, and that the agencies now modifying the surface have been alike, in every respect, in all past time. It is true that no restriction is placed on the extent of the changes, but such prolonged time is insisted on for their accomplishment as to destroy the value of the concession. Not that time is in itself a difficulty, but a time rate, assumed on very insufficient grounds, is used as a master key, whether or not it fits, to unravel all difficulties. What if it were suggested that the brick-built Pyramid of Hâwara had been laid brick by brick by a single workman? Given time, this would not be beyond the bounds of possibility. But Nature, like the Pharaohs, had greater forces at her command to do the work better and more expeditiously than is admitted by uniformitarians.
, On the other side, physicists would lead us to suppose that those great movements of the earth's crust, with which we are all familiar in the form of high mountain and continental upheavals in the earlier stages of the earth's history, were impossible in those times which more immediately approached our own. They maintain that if the earth is not solid throughout, its outer crust at least must have now attained a thickness estimated to vary from eight hundred to twenty-five hundred miles, and is so rigid that we are forced to believe that for a long preceding period it must have been in a state of comparatively stable equilibrium. This, however, would have rendered the great earth movements, considered by geologists to have continued up to the threshold of our own times, impossible. And to this finding the physicists would have geological speculations conform. At the same time, judging, among other reasons, from the rate of cooling of hot solid bodies, they would assign a much shorter term to the earth's history since it became habitable than is compatible with the views of the uniformitarian school of geologists. The one side counts in round numbers upon some three hundred million years; the other sees no reason to go beyond fifteen to twenty million years —a term, in our humble opinion, much more probable than the other.
On another point our two allies (allies in the sense of working at the same subject) are in irreconcilable antagonism. The physicists tell us that uniformity of action in all time is impossible, while the uniformitarians say that such a shortening of geological time as would follow on the acceptance of the physical argument is against all geological experience. Not only do these opinions clash, but those also concerning the rigidity of the earth and the thickness of its crust are widely divergent. None of these contentions can, however, be disregarded, for we must all recognize the importance of considering the question from every point of view. The argument in favor of uniformity of action has been put before us with so much skill and ability, and possessing as it does the charm of an infallible faith, that uniformitarianism has become the accepted doctrine of the dominant school of geology. Besides, within certain limits and in certain lights, the arguments of the uniformitarian and of the physicist might hold good—that is to say, if we would restrict the deductions of the former to the recent period, and could adopt the propositions of the latter. Our part, however, is to see whether their conclusions agree—not with their respective assumptions, but with the geological evidence: for no conclusions can be accepted that do not meet with the full concurrence of all the copartners interested in the result, and without respect for their mutual claims progress is not possible. The geologist must attend to the claims of the physicist, and the physicist ought not to overlook those of the geologist. How then stands the case?
With regard to the geological problem, we are told by the uniformitarians that the forces acting on the surface of the globe have been in all past times the same, both in kind and degree, as those now in operation. On those grounds they have proceeded to estimate, first, the time required for mountain and continental elevation; secondly, the rate of erosion of the valleys, and of the denudation or lowering of the land. Their conjecture is that our limited experience of two thousand to three thousand years has sufficed to furnish us with instances of all the various vicissitudes and changes that the earth has undergone during the illimitable past a generalization incompatible with what is known of the evolution of the earth, and in contradiction to their own premises. For even geologists who recognize no change admit the original molten state of the globe. This of itself involves, in the cooling of the mass, the intervention of stresses and strains, with all their consequences, which render it inconceivable that there was nothing in all those stages of the earth's history beyond what our limited experience has brought us in contact with.
But although the assumption of the uniformitarians on. the question of degree may be disputed, that on the question of kind admits of no dispute. That rivers excavate and currents distribute the excavated materials, and that the land is mobile and subject to changes of level, no one will contest. The point of contention is the rate at which these operations and changes proceeded formerly as compared with the rate at the present day. The many observations made on the erosive and transporting power of rivers, and on the movements and waste of the land, are admirable in so far as they apply to the silting up of ports, the recession of the coast, and the reclamation of marsh lands; but, though valuable to the engineer, they are misleading to the geologist. They furnish him, it is true, with standards applicable to present changes, and indicate the method in which the erosive power of the rivers and seas has acted in all time, but they give no measure of the amount and rate of work they did at different periods. Nevertheless, knowing what at present is accomplished by their means, it is' reasonable to judge, by ascertaining what their agency accomplished in former days, of the difference in the forces in operation at the several periods. Those forces have to be estimated by the work done in the past, and not by any fixed rate founded upon present work.
Few geologists would, we presume, contest this position; notstanding which, and though many now profess a modified uniformitarianism, the old lines of argument still, with few exceptions, prevail, and the concessions made are more apparent than real, or are of little value. In our opinion, no partial concession can be entertained on the question of degree. It must be an unconditional surrender; for, in contradistinction to method, or manner, where we are on common ground, no common scale on the question of degree is possible in judging of the past by comparison with the present.
As an example of the present position, we may take one argument as presented by the advocates of the uniformitarian school. The observations on the transporting power of the large rivers of the world have shown that the quantity of sediment carried down by them to the sea is, according to one of their estimates, such as would suffice to lower the level of the land about one foot in six thousand years, or about a thousand feet in six million years. Exception might be taken to this estimate in that no account is taken of the calcareous matter removed in solution, which, in fact, is not far from the quantity of insoluble matter carried down mechanically. Let that pass. This measure, or one approximate to it, has been very generally accepted, and is in common use. Hence those geologists, proceeding solely on the assumed postulate, and not attaching due weight to other considerations, have, it seems to us, placed the later Quaternary times at far too great a distance from the present. In the same way, the rate at which the elevation of the land took place having been estimated on the mean of two and a half feet in a century, would, if that scale were accepted, manifestly push back to a very remote distance even later geological changes.
The importance of determining these points more accurately became more evident when it was discovered that man existed with the extinct mammalia; and therefore upon the solution of the time-rate problem depended the determination of the antiquity of man upon the earth. Various have been the attempts since made; but, as they have almost all been made upon measurements based on the above-named scales, they necessarily involved a very free use of time. For long, geologists had held to the belief, prevailing half a century ago, that man could not have existed on the earth for more than five to six thousand years. When evidence was given, and at last accepted, to prove a higher antiquity, the uniformitarians were placed in the difficulty of proving too little or too much. If they adopted a short chronology, it would clash with the corner stone of their belief as to the age of the Quaternary deposits; if, on the other hand, they retained their belief in the great length of time they held to be necessary for the formation of the post or later glacial deposits, they would have to assign to man an antiquity which would clash sorely not only with their own previous belief, but also with that held on various grounds by other geologists and anthropologists.
The fetich of uniformity prevailed, the uniformitarians made volte-face to their former contention, and hesitated not to claim for man an antiquity going on for a million years. One old friend of ours, in a public lecture, even put in a claim for two millions, heedless of the cries of his unprepared audience to remind him of the rights of Adam. At a loss to prove their case by independent geological evidence, they found an unexpected ally in a novel and ingenious astronomical hypothesis, which apart from its connection with geology we will not contest. The object of the hypothesis was to show that there had been cycles, in which at times the position of the earth in its orbit was such as would cause a great lowering of the terrestrial temperature and give rise to recurring glacial periods. Here were offered the definite measures that geology failed to furnish, and which tallied too well with the time needed by the uniformitarians to be neglected. It was therefore eagerly adopted, and has since been prominent in geological literature. That the hypothesis, however, is not in accordance with the facts of geology has been abundantly shown both in America and in this country; nevertheless the belief prevails. The result is that, as the last of these astronomical periods was calculated to have commenced two hundred and fifty thousand years and to have ended eighty thousand years ago, these numbers have become stereotyped as those of the beginning and the end of the Glacial period.
The able author of this hypothesis, in his attempt to reconcile geological and astronomical time, built his geological argument upon the rate of erosion of rivers at the present time, as held by the uniformitarian. Nevertheless, an observation of his own, that must be indorsed by all geologists, whatsoever their creed, shows the fallacy of adopting the rates of the present day as measures for the past, for he remarks: "If the rate of denudation he at present so great, what must it have been during the Glacial period? It must have been something enormous." Very true, yet the argument proceeds as before. With the admission here made, how is it possible to adopt a scale admitted by its advocate to be subject to such variation? Its retention only serves to divert the real issue and stay inquiry.
Another objection to this chronology is that it fixes the date of the disappearance of palæolithic man and the Quaternary fauna at a distance of eighty thousand years from our own times. Of these eighty thousand years, we can account for ten or twelve thousand during which neolithic and recent man has been in occupation of the land; but this leaves some seventy thousand years unaccounted for. Unable satisfactorily to show on geological grounds the need of so great an interval between the end of the Quaternary period and the present time, the uniformitarians find a more colorable defense on biological grounds. They point, in a manner we do not quite understand, to the circumstance that with the close of the Post-glacial period a number of the animals then living disappear from the scene, and contend that for the dying-out of so many species long ages must have been required. Had they been able to show the working of evolution in the coming in of new species by descent from the extinct species, or of change in the contemporary species still living, their argument could not be gainsaid. But there is no question of evolution. The mammoth and woolly rhinoceros disappeared for good; the reindeer, musk ox, and glutton were driven to northern latitudes, and there still survive unchanged; while the horse, ox, red deer, wolf, fox, badger, hare, and others remain on with us without variation of species. The extraordinary change of climate which then took place is quite sufficient to account for such changes as these, which are chiefly of those of faunal distribution, having been effected in a measurable length of time, instead of needing the vastly long period mentioned. This length of time could hardly have failed to involve more extensive changes in the species, even without the aid of the physical changes which then took place, than are apparent in the species now existing. There is, in fact, no sufficient evidence either geological or biological to show the need of the long interval assumed. On the contrary, there is every reason to believe that it did not exist, but that palæolithic man and his companions came down to within some ten to twelve thousand years of our times. We can not suppose that either man or geological work would have remained stationary during seventy thousand years, and yet that is the conclusion we should be driven to adopt. Are we to be debarred from pursuing these inquiries by a hypothesis having no better foundation, and involving such unquestionable difficulties?
Another barrier to inquiry is the postulate which would fix the rate of upheaval of the land during geological periods upon observations based—not upon the experience of even two or three thousand years—but upon observations which do not extend beyond two centuries. These observations have shown, as put by uniformitarians, that the mean rate of elevation of the coasts of Norway and Sweden has been during that time two and a half feet in a century, and this scale has been accepted and employed unhesitatingly as a safe and sure basis for calculation of geological time. The determination of a secular rise of the land is of itself an interesting fact, as settling the question of a retained mobility in the earth's crust; but it is quite insufficient, even if it were applicable, to establish a definite rate, not only for the past but even for the present. It is not a mean rate that is wanted. No upheaval can be otherwise than local and graduated. The extremes are what is needful. No engineer would take the mean delivery of a river as the measure to be depended upon for a water supply. It is the limit in both directions, or the minimum and maximum quantities, that are essential. To know what earth movements can still effect, we should at least take the maximum rate, which amounts in the above case, at the North Cape, to five feet in the century, or double the measure of the mean adopted by uniformitarians.
If also, in calculating the present rate of elevation of the land, the mean rate along the whole length of the axis is adopted, the same rule should at least be applied to elevations of past periods, and the time should not be estimated by the height of any one point, as that may prove to be more or less in excess of the mean. Thus, for example, the Westleton marine shingle is found in Buckinghamshire at a height of six hundred feet. Estimating this upheaval at the rate of two and a half feet in a century, the uniformitarian would put in a claim for twenty-four thousand years. But this bed, as it trends eastward, is met with at gradually lower levels, until in Suffolk it falls to the sea level. A mean of three hundred feet should therefore be taken, with a corresponding shorter time-term of twelve thousand years; or are we to ignore any interval of time and to look only at the beds on the coast where they are consecutive? From every point of view such estimates must be worthless.
More than this, the very leaders of the belief that the average rate of motion does not exceed that above named allow that "the average rate proposed is a purely arbitrary and conjectural one." It is admitted also that it is not improbable that during the last four hundred years there has been a still faster rate in high northern latitudes. Not only, however, is the half measure adopted, but the warning that higher measures exist is neglected. When therefore the mean is applied to determine the length of time required to effect such elevations as that of the marine shell bed on Moel Tryfaen, fourteen hundred feet above sea level and of late Quaternary age, uniformitarians are obliged to ask for a term of fifty-six thousand if not eighty thousand years. Should the case of Moel Tryfaen be objected to as uncertain, there are still the unquestioned raised beaches of Norway and Sweden, which are from two hundred to six hundred feet above the sea level, and of still more recent date. These, on the same estimate, would have taken for their upheaval some eight thousand to twenty-four thousand years. We need not, however, pursue this subject further. The very admissions of the advocates of tho two above-named measures of time, based upon present rates of denudation and of elevation, show how untenable their conclusions are.
Such observations, howsoever useful and suggestive, are in fact futile so far as regards their application to former rates of upheaval, and needlessly play with time. If we could suppose that the causes which produced those movements had' always acted with the same degree of energy, the reasoning would hold good; but, as that regularity depends upon the stresses to which the earth's crust has been exposed at any particular time, the effects must have varied in proportion as the stresses varied. With a cooling globe it could not have been otherwise. What those movements of the past were, and what their duration, must therefore be judged of by other circumstances and on surer data.
We trust we have now said enough to show upon how insecure a basis the uniformitarian measures of time and change stand. They have probably done more to impede the exercise of free inquiry and discussion than any of the catastrophic theories which formerly prevailed. The latter found their own cure in the more accurate observation of geological phenomena and the progress of the collateral sciences; but the former hedge us in by dogmas which forbid any interpretation of the phenomena other than that of fixed rules which are more worthy of the sixteenth than of the nineteenth century. Instead of weighing the evidence and following up the consequences that should ensue from the assumption, too many attempts have been made—not unnaturally by those who hold this faith—to adjust the evidence to the assumption. The result has been strained interpretations framed to meet one point, but without sufficient regard for the others. We repeat that we would not for a moment contend that the forces of erosion, the modes of sedimentation, and the methods of motion, are not the same in kind as they have ever been, but we can never admit that they have always been the same in degree. The physical laws are permanent; but the effects are conditional and changing, in accordance with the conditions under which the law is exhibited.
Such are the barriers which seem to us seriously to retard the advance in one direction of an important branch of theoretical geology, while in another it is fronted by the stern rules of an apparently definite calculation.
We must ask to be forgiven if we can not accept the conclusions of physicists respecting the extreme rigidity of the earth and the immobility of the crust as conclusive. That the rigidity is now very great—as great, we will admit for argument's sake, as if the globe were of glass or steel—may be as asserted, but that conclusion can only be accepted in so far as it conforms to the facts of geology. Were the data on which the conclusion is based fixed and positive, like those on which the laws of gravitation and light are established, there would be nothing for the geologist to do but to bow to the decision of the physicist, and, if possible, revise his work. But in this case the tidal observations, on which the calculations of rigidity are mainly based, are of such extreme delicacy that, failing as the hypothesis does to satisfy the requirements of geology, the geologist may be excused for his dissent, pending further inquiry. Should this tend to confirm the extreme rigidity of the globe, we must seek for some explanation of earth movements consistent with that rigidity. It is indisputable that up to the latest geological period—that touching on our own times—the mobility of the crust was very considerable, for the raised beaches of Europe and of the Mediterranean prove conclusively that in that period extensive tracts were raised at intervals to heights of from ten to six hundred feet or more above their former levels. It is difficult to conceive that a globe, of which the crust was then so mobile, could have acquired, in the comparatively short interval between the latest of the beaches and our own time, so great a rigidity as to be practically immobile.
For similar reasons the conjoint conclusion that the crust of the earth is not less than from a thousand to twenty-five hundred miles thick is open to question. We can not imagine that a crust of that enormous thickness could, in such recent geological times, have possessed so great a flexibility as is indicated by the movements we have referred to. Independently of that improbability, there are certain geological facts which are inexplicable on that assumption. Volcanic phenomena would be unintelligible; for vents traversing that thickness of solid rock could hardly be kept open owing to the cooling which the lava in its ascent would undergo. The rock fragments ejected during explosions are also those of rocks which lie at no great depth, while, with the increase of temperature in descending beneath the surface, there is every reason to suppose that at a depth to be measured by tens, and not by hundreds of miles, the immediate underlying magma at least is in a state of plasticity such as would allow of comparatively free movements of the crust. Again, surely, if the crust were so thick, we might expect to find, when that crust was broken and its edges thrust up by compression or protrusion of the igneous rocks, that some indications of that enormous thickness should be exhibited; but none such are forthcoming. Whatever may be the state of the nucleus, there is nothing geologically to indicate, as some physicists also have contended on other grounds, that the outer crust of the earth is more than from about twenty to thirty miles thick. The effective rigidity will therefore, if it be necessary, have to be explained in some other manner than that of a comparatively solid globe or of a crust of enormous thickness.
We are thus brought face to face with apparently irreconcilable opinions. That they admit of adjustment there can be no doubt, but it must be by mutual understanding. How it is to be effected is a problem for the future.
These, briefly, are the barriers which restrict inquiry on many important questions. On the side of the uniformitarians, it is assumed that every position must be reduced to a fixed measure—where fixity is not possible—of time and speed; and, on that of the physicists, geologists are gently reminded that the subject is outside their immediate sphere of inquiry, in a way somewhat suggestive of "the closure."
It would be an unfortunate day for any science to have free discussion and inquiry barred by assumed postulates, and not by the ordinary rules of evidence as established by the facts, however divergent the conclusions to which those facts lead may be from the prevailing belief. In any case it must be remembered that no hypothesis can be true which does not satisfy the conditions both of the geological phenomena and of the physical laws. The foregoing remarks are intended to apply mainly to questions connected with the more recent geological periods. The older epochs have happily been treated as beyond the barriers, and consequently have enjoyed and made good use of their greater freedom. It is to be hoped that, when the phenomena of these later periods are judged of by the evidence of facts rather than by rules, they will receive more independent interpretations—interpretations that may escape the dwarfing influence of uniformitarianism.—Nineteenth Century.
