Popular Science Monthly/Volume 56/March 1900/A Century of Geology II
| A CENTURY OF GEOLOGY. |
By Prof. JOSEPH LE CONTE.
[Concluded.]
THE AGE OF THE. EARTH.
UNTIL almost the beginning of the present century the general belief in all Christian countries was that not only the earth and man, but the whole cosmos, began to exist about six thousand to seven thousand years ago; furthermore, that all was made at once without natural process, and have remained substantially unchanged ever since. This is the old doctrine of the supernatural origin and substantial permanency of the earth and its features. Among intelligent and especially scientific men this doctrine, even in the eighteenth century, began to be questioned, although not publicly; for in 1751 Buffon was compelled by the Sorbonne to retract certain views concerning the age of the earth, published in his Natural History in 1749.[1] Remnants of the old belief lingered even into the early part of the present century, and may even yet be found hiding away in some of the remote corners of civilized countries. But with the birth of geology, and especially through the work of Hutton in Scotland, Cuvier in France, and William Smith in England, the much greater—the inconceivably great—antiquity of the earth and the origin of its present forms, by gradual changes which are still going on, was generally acknowledged. Indeed, as already said, this is the fundamental idea of geology, without which it could not exist as a science.
Geology has its own measures of time—in eras, periods, epochs, ages, etc.—but it is natural and right that we should desire more accurate estimates by familiar standards. How old, then, is the earth, especially the inhabited earth, in years? Geologists have attempted to answer this question by estimates based on the rates of sedimentation and erosion, or else on the rate of changes of organic forms by struggle for life and survival of the fittest. Physicists have attempted to answer the same question by calculations based on known laws of dissipation of energy in a cooling body, such as the sun or the earth. The results of the two methods differ widely. The estimates of the geologists are enormous, and growing ever greater as the conditions of the problem are better understood. Nothing less than several hundred million years will serve his purpose. The estimates of the physicists are much more moderate, and apparently growing less with each revision. The latest results of King and Kelvin give only twenty to thirty millions[2]. This the geologist declares is absurdly inadequate. He can not work freely in so narrow a space—he has not elbow room.
The subject is still discussed very earnestly, but with little hope of definite conclusion. One thing, however, must be remarked. Both parties assume—the geologist tacitly, the physicist avowedly—the nebular hypothesis of the origin of the solar system, and therefore the early incandescent fluid condition of the earth as the basis of all his reasonings. Now, while this is probably the most reasonable view, it is not so certain that it can be made the basis of complex mathematical calculation. There is a possible alternative theory—viz., the meteoric theory—which is coming more and more into favor. According to this view, the planets may have been formed by aggregation of meteoric swarms, and the heat of the earth produced by the collision of the meteors in the act of aggregation. According to the one view (the nebular), the heat is all primal, and the earth has been only losing heat all the time. According to the other, the aggregation and the heating are both gradual, and may have continued even since the earth was inhabited. According to the one, the spendthrift earth wasted nearly all its energy before it became habitable or even a crust was formed, and therefore the habitable period must be comparatively short. According to the other, the cooling and the heating, the expenditure and the income, were going on at the same time, and therefore the process may have lasted much longer.
The subject is much too complex to be discussed here. Suffice it to say that on this latter view not only the age of the earth, but many other fundamental problems of dynamical geology, would have to be recalculated. The solution of these great questions must also be left to the next century. In the meantime we simply draw attention to two very recent papers on the subject—viz., that of Lord Kelvin[3], and criticism of the same by Chamberlin[4].
ANTIQUITY AND ORIGIN OF MAN.
Even after the great antiquity of the earth and its origin and development by a natural process were generally accepted, still man was believed, even by the most competent geologists, to have appeared only a few thousand years ago. The change from this old view took place in the last half of the present century—viz., about 1859—and, coming almost simultaneously with the publication of Darwin's Origin of Species, prepared the scientific mind for entertaining, at least, the idea of man's origin by a natural process of evolution.
Evidences of the work of man—flint implements, associated with the bones of extinct animals and therefore showing much greater age than usually accepted—had been reported from time to time, notably those found in the river Somme by Boucher de Perthes. But the prejudice against such antiquity was so strong that geologists with one accord, and without examination, pooh-poohed all such evidence as incredible. It was Sir Joseph Prestwich who, in 1859, first examined them carefully, and published the proofs that convinced the geological world that early man was indeed contemporaneous with the extinct animals of the Quaternary period, and that the time must have been many times greater than usually allowed[5].
Since that time confirmatory evidence has accumulated, and the earliest appearance of man has been pushed back first to the late glacial, then to the middle glacial, and finally, in Mr. Prestwich's Plateau Gravels, to the early glacial or possibly preglacial times.
Still, however, in every case earliest man was unmistakably man. No links connecting him with other anthropoids had been found. Very recently, however, have been found, by Du Bois, in Java, the skull, teeth, and thigh bone of what seems to be a veritable missing link, named by the discoverer Pithecanthropus erectus. The only question that seems to remain is whether it should be regarded as an ape more manlike than any known ape, or a man more apelike than any yet discovered. The age of this creature was either latest Pliocene or earliest Quaternary.
BREAKS IN THE GEOLOGICAL RECORD AND THEIR SIGNIFICANCE.
From the earliest times of geologic study there have been observed unconformities of the strata and corresponding changes in the fossil contents. Some of these unconformities are local and the changes of organic forms inconsiderable, but sometimes they are of wider extent and the changes of life system greater. In some cases the unconformity is universal or nearly so, and in such cases we find a complete and apparently sudden change in the fossil contents. It was these universal breaks that gave rise to the belief in the occurrence of violent catastrophes and corresponding wholesale exterminations and re-creations of faunas and floras.
It is evident, however, on a little reflection, that every such unconformity indicates a land period at the place observed, and therefore a time unrecorded in strata and fossils at that place—i. e., a lost interval—certain leaves missing from the book of time. And if the unconformity be widespread, the lost interval is correspondingly great. It is therefore probable that change of species went on slowly and uniformly all the time, although not recorded at that place. Intermediate strata may be and often are found elsewhere, and the supposed lost interval filled. The record was continuous and the changes uniform, but the record is not all found in one place. The leaves of the book of Time are scattered here and there, and it is the duty of the geologist to gather and arrange them in proper order, so that the record may read continuously.
This is the uniformitarian view, and is undoubtedly far truer than the catastrophic. But the objection to it is that in the case of very widespread unconformities, such as occurred several times in the history of the earth, the changes of organisms are so great that if the rate of change was uniform the lost interval must have been equal to all the rest of the history put together. Therefore we are compelled to admit that in the history of the earth there have been periods of comparative quiet (not fixedness) during which evolutionary changes were slow and regular, and periods of revolution during which the changes were much more rapid, but not catastrophic. This is exactly what we ought to expect on the idea of gradual evolution of earth forms by secular cooling, for in the gradual contraction of the earth there must come times of general readjustment of the crust to the shrinking nucleus. These readjustments would cause great changes in physical geography and climate, and corresponding rapid changes in organic forms. In addition to this, the changes in physical geography and climate would cause extensive migrations of species, and therefore minglings of faunas and floras, severer struggles of competing forms, and more rapid advance in the steps of evolution. Among these changes of organic forms there would arise and have arisen new dominant types, and these, in their turn, would compel new adjustment of relations and still further hasten the steps of evolution. Such changes, whether geographic, or climatic, or organic, would not be simultaneous all over the earth, but propagated from place to place, until quiet was re-established and a new period of comparative stability and prosperity commenced.
This view is a complete reconciliation of catastrophism and uniformitarianism, and is far more rational than either extreme.
Critical Periods in the History of the Earth.—Such periods of rapid change may well be called critical periods or revolutions. They are marked by several characteristics: (1) By widespread oscillations of the earth's crust, and therefore by almost universal unconformities. (2) By widespread changes of physical geography, and therefore by great changes in climate. (3) By great and widespread changes in organic forms, produced partly by the physical changes and partly by the extensive migrations. (4) By the evolution of new dominant types, which are also the cause of extensive changes in species. (5) Among the physical changes occurring at these times is the formation of great mountain ranges. The names of these critical periods or revolutions are often taken from the mountain range which form their most conspicuous features.
There have been at least four of these critical periods, or periods of greatest change: (1) The pre-Cambrian or Laurentide revolution; (2) the post-Paleozoic or Appalachian; (3) the post-Cretaceous or Rocky Mountain; (4) the post-Tertiary or glacial revolution.
Now, as these critical periods separate the primary divisions of time—the eras—it follows that the Present—the Age of Man—is an era. It may be called the Psychozoic Era. These views have been mainly advocated by the writer of this sketch, but I believe that, with perhaps some modification in statement, they would be accepted by most geologists as a permanent acquisition of science[6].
GEOLOGICAL CLIMATES.
Attention was first drawn to this subject by the apparently unique phenomena of the Glacial epoch.
For nearly a century past Alpine glaciers, their structure, their mysterious motion, and their characteristic erosive effects, have excited the keenest interest of scientific men. But until about 1840 the interest was purely physical. It was Louis Agassiz who first recognized ice as a great geological agent. He had long been familiar with the characteristic marks of glacial action, and with the fact that Alpine glaciers were far more extensive formerly than now, and had, moreover, conceived the idea of a Glacial epoch—an ice age in the history of the earth. With this idea in his mind, in 1840 he visited England, and found the marks of glaciers all over the higher regions of England and Scotland. He boldly announced that the whole of northern Europe was once covered with a universal ice sheet. A few years later he came to the United States, and found the tracks of glaciers everywhere, and again astonished the world by asserting that the whole, northern part of the North American continent was modeled by a moving ice sheet. This idea has been confirmed by all subsequent investigation, especially here in America. But it would be strange, indeed, if the cold of the Glacial epoch should be absolutely unique. Attention was soon called to similar marks in rocks of other geological periods, especially in the Permian of the southern hemisphere. This opened up the general question of geological climates and their causes.
Perhaps no subject connected with the physics of the earth is more obscure and difficult than this. The facts, as far as we know them, are briefly as follows: (1) All the evidence we have point to a high, even an ultra-tropical, climate in early geological times; (2) all the evidence points to a uniform distribution of this early high temperature, so that the zonal arrangement of temperatures, such as characterizes present climates, did not then exist; (3) temperature zones were apparently first introduced in the late Mesozoic (Cretaceous) or early Tertiary times, and during the Tertiary the colder zones were successively added, until at the end there was formed a polar ice-cap as now.
Thus far all might be explained by progressive cooling of the earth and progressive clearing of the atmosphere of its excess CO2 and aqueous vapor. But (4) from time to time (i.e., at critical periods) there occurred great oscillations of temperature, the last and probably the greatest of these being the Glacial period. The cause of these great oscillations of temperature, and especially the cause of the glacial climate, is one of the most interesting and yet one of the obscurest and therefore one of most hotly disputed points in geology. Indeed, the subject has entered into the region of almost profitless discussion. We must wait for further light and for another century. Only one remark seems called for here. It is in accordance with a true scientific method that we should exhaust terrestrial causes before we resort to cosmical. The most usual terrestrial cause invoked is the oscillation of the earth's crust. But recently Chamberlin, in a most suggestive paper[7], has invoked oscillations in the composition of the atmosphere, especially in its proportion of CO2, as the immediate cause, although this in turn is due to oscillations of the earth's crust.
THE NEW GEOLOGY.
Heretofore the geological history of the earth has been studied only in the record of stratified rocks and their contained fossils. But in every place there have been land-periods in which, of course, erosion took the place of sedimentation. This kind of record is very imperfect, because there are no fossils. Until recently no account was taken of these erosion-periods except as breaks of indefinite length in the record—as lost intervals. But now, and mainly through the work of American geologists, interpretation of these erosion-periods has fairly commenced, and so important has this new departure in the study of geology seemed to some that it has been hailed as a new era in geology, connecting it more closely with geography. Heretofore former land periods were recognized by unconformities and the amount of time by the degree of change in the fossils, but now the amount of time is estimated in existing land surfaces by topographic forms alone. This idea was introduced into geology by Major J. W. Powell, and has been applied with success by William Morris Davis, W J McGee, and others.
The principle is this: Land surface subject to erosion and standing still is finally cut down to gently sweeping curves, with low, rounded divides and broad, shallow troughs. Such a surface is called by Davis a Peneplain. Such a peneplain is characteristic of old topography. If such a surface be again lifted to higher level, the rivers again dissect it by ravines, which are deep and narrow in proportion to the amount and rate of the uplift. If the land again remains steady, the sharply dissected surface is again slowly smoothed out to the gentle curves of a peneplain. If, on the contrary, the surface be depressed, the rivers fill up the channels with sediment which, on re-elevation, is again dissected. Thus the whole ontogeny of land surfaces have been studied out, so that their age may be recognized at sight.
Thus, while heretofore the more recent movements of the crust were supposed to be readable only on coast lines and by means of the old sea strands, now we read with equal ease the movements of the interior by means of the physiognomy of the topography, and especially the structure of the river channels. Moreover, while heretofore the history of the earth was supposed to be recorded only in stratified rock and their contained fossils, now we find that recent history is recorded and may be read also in the general topography of the land surfaces. Geography is studied no longer as mere description of earth forms, but also as to the causes of these forms, no longer as to present forms, but also as to the history of their becoming. Thus geography, by its alliance with geology, has become a truly scientific study, and as such is now introduced into the colleges and universities. It is this alliance with geology which has caused the dry bones of geographic facts to live. It is this which has created a soul under the dry "ribs of this death." This mode of study of the history of the earth has just commenced. How much will come of it is yet to be shown in the next century.
In this connection it is interesting to trace the effect of environment on geological reasonings in different countries. Heretofore, especially in England, what we have called peneplains were usually attributed to marine denudation—i. e., to cutting back of a coast line by constant action of the waves, leaving behind a level submarine plateau, which is afterward raised above sea level and dissected by rivers. American geologists, on the contrary, are apt to regard such level surfaces as the final result of aerial degradation or a base level of rain and river erosion. The same difference is seen in the interpretation of glacial phenomena. Until recently, English geologists were inclined to attribute more to iceberg, Americans more to land ice. Again, in England coast scenery is apt to be attributed mainly to the ravages of the sea, while in America we attribute more to land erosion combined with subsidence of the coast line. In a word, in the tight little sea-girt island of Great Britain, where the ravages of the sea are yearly making such serious inroads upon the area of the land, it is natural that the power of the sea should strongly affect the imagination and impress itself on geological theories, and tend perhaps to exaggeration of sea agencies, while the broad features of the American continent and the evidences of prodigious erosion in comparatively recent geological time tend to the exaggeration of erosive agency of rain and rivers. These two must be duly weighed and each given its right proportion in the work of earth sculpture.
PALEONTOLOGY.
Paleontology at first attracted attention mainly by the new and strange life forms which it revealed. It is the interest of a zoölogical garden. This interest is of course perennial, but can hardly be called scientific. Geology at first was a kind of wonder book.
Next fossils, especially marine shells, were studied as characteristic forms denoting strata of a particular age. They were coins by which we identify certain periods of history. They were "medals of creation." It was in this way chiefly that William Smith, the founder of English stratigraphic geology, used them. It was in this way that Lyell and all the older geologists, until the advent of evolution, were chiefly interested in them.
It was Cuvier, the great zoölogist and comparative anatomist, who, in the beginning of the present century, first studied fossils, especially mammalian fossils, from the zoological point of view—i. e., as to their affinities with existing animals. Cuvier's studies of the vertebrates of the Paris basin may be said to have laid the foundation of scientific paleontology from this point of view.
Thenceforward two views of paleontology and two modes of study gradually differentiated from one another, the one zoölogical, the other geological. In the one case we study fossils in taxonomic groups—i.e., as species, genera, families, orders, etc.—and trace the gradual evolution of each of these from generalized forms to their specialized outcomes, completing as far as possible the genetic chain through all time. In the other we study fossils in faunal groups, as successive geological faunas, and the geographic diversity in each geological period—i.e., the evolution of geologic faunas and the causes of geographic diversity in each. In a word, we study the laws of distribution of faunas in time geologically and in space geographically, and the causes of these laws in each case. The first is strictly a branch of zoölogy and botany, and we leave it to these specialists. The second alone belongs properly to geology. In this purely geologic paleontology, as seen from its scope given above, there are many questions of widest philosophical interest which are only now attracting the attention they deserve. I only touch lightly two which have been brought forward in these very last years of the century.
I. General Laws of Faunal Evolution.—The evolution of the organic kingdom from this strictly geological point of view may be briefly formulated as follows:
1. Throughout all geological time there has been a general movement upward and onward, as it were abreast, everywhere. If this were all, there would be only geological progress, but no geographical diversity. Geological history would be the same everywhere. A time horizon would be easily determined by identity of fossil species. This we know is not true. Therefore there are other elements besides this.
2. In different countries, isolated from one another and under different conditions, evolution takes different directions and different rates, producing geographical diversity in each geological period. This diversity increases with time as long as the isolation continues. If this were all, the geographical diversity by continued divergence would have become so great that it would be impossible even approximately to determine any geological horizon. The history of each country must be studied for itself. A general history of the earth would be impossible. But this also is not true. There is therefore still another element.
3. From time to time, at long intervals—i.e., critical periods—there are widespread readjustments of the crust to internal strain, determining changes of physical geography and of climate, and therefore wide migrations of species with mingling and conflict of faunas. This would produce more rapid movement of evolution, but at the same time more or less complete obliteration of geographical diversity.
4. After these periods of migrations and minglings there would be re-isolations in new localities, and the process of diversification would recommence and increase as long as the isolation continues.
The last of these critical periods of migrations and minglings and struggles for life among competing species was the Glacial epoch or ice age. Therefore the present geographical distribution of species was largely determined by the extensive migrations of that time.
II. Cosmopolitan and Provincial Faunas.—There are apparently in the history of the earth periods of widespread or cosmopolitan faunas, alternating with localized or provincial faunas. The cosmopolitan periods are usually times of prevalence of limestones or organic sediments, and the fossils are very abundant. The provincial periods are usually characterized by sandstones and shales or mechanical sediments, and are comparatively poor in fossils. Moreover, it is believed that the cosmopolitan limestone periods are oceanic periods—i.e., periods of wide oceans and lower and smaller continents and little erosive activity, while the sandstone periods, characterized by provincial faunas, are periods of higher and larger continents, and therefore of great erosion and abundant mechanical sedimentation.
Now, according to Chamberlin, these remarkable alternations are due to oscillations of the crust, in which the continents are alternately lifted and depressed. It must be remembered that abyssal faunas are almost unknown among fossils. This is the necessary result of substantial permanency of oceanic basins. The whole geological record is in shallow-water faunas. These shallow waters are along continental shore lines and in interior continental seas. According to Chamberlin again, during a period of continental depression all the flat continental margins are submerged, forming broad submarine platforms, and the lower interior portions of the continents are also submerged, forming wide and shallow interior seas. Under these conditions continental waste, and therefore sand and clay sediments, are reduced to a minimum. Life, animal and vegetal, abounds, and therefore much limestone is formed. The oceans are widely connected with one another, and therefore the faunas are widespread or cosmopolitan. During the period of elevation, on the contrary, the continents are extended to the margin of the deep oceanic basins, the broad, shallow submarine platforms are abolished, the interior seas are also abolished, the shallow-water areas are reduced to isolated bays, and their faunas are peculiar or provincial. Also, elevated and enlarged continents give rise to maximum erosion, and therefore abundant sediments of sandstone and clay, and comparative poverty of life and therefore of limestone. Chamberlin also gives reasons why the oceanic periods should be warm, humid, equable in temperature, and the atmosphere highly charged with CO2, and therefore highly favorable to abundant life, both vegetal and animal, while land periods would be drier and cooler, the atmosphere deficient in CO2, and therefore cold from that cause and in many ways unfavorable to abundant life.
These extremely interesting views, however, must be regarded as still on trial, as a provisional hypothesis to be sifted, confirmed, or rejected, or in any case modified, in the next century.
Lastly, it is interesting to note the ever-increasing part taken by American geologists in the advance of this science. There has been through the century a gradual movement of what might be called the center of gravity of geological research westward, until now, at its end, the most productive activity is here in America. This is not due to any superiority of American geologists, but to the superiority of their opportunities. Dana has well said that America is the type continent of the world. All geological problems are expressed here with a clearness and a simplicity not found elsewhere. We must add to this the comparative recency of geological study in this rich field. In Europe the simpler and broader problems are already worked out, and all that remain are difficult problems requiring much time. In America, on the contrary, not only are all problems expressed in simpler terms, but many great and broad problems are still awaiting solution. For these reasons the greatest activity in research, and the most rapid advance during the next century, will probably be here in America.
- ↑ Lyell's Principles of Geology, eighth edition, p. 41.
- ↑ Clarence King, American Journal Of Science, pp. 45-51, 1893; Kelvin, Science, vol. ix, p. 665, 1899.)
- ↑ Science, vol. ix, p. 665, 1899.
- ↑ Ibid., p. 889, and vols. x and xi, 1899
- ↑ Life and Letters of Sir Joseph Prestwich, pp. 124 et seq.
- ↑ Critical Periods, etc., American Journal of Science, vol. xiv, p. 99, 1877; Bulletin of the Geological Department of the University of California, vol. i, No. 11, 1895.
- ↑ Journal of Geology, vol. vi, p. 597, 1898, and vol. vii, p. 545, 1899.