Popular Science Monthly/Volume 1/June 1872/Sir Charles Lyell
|SIR CHARLES LYELL.|
SIR CHARLES LYELL is now seventy-five years old. He took his degree in Oxford, in 1821, and commenced the study of law, but, influenced by Dr. Buckland, he soon left it for that of geology. His first original papers on this subject were published in 1826, and the first volume of his great work, the "Principles of Geology," was issued in 1830. He came to the United States in 1841, and again in 1845, and visited many localities for the purpose of scientific observation. He became president of the Geological Society in 1836, and again in 1850, and was honored with knighthood, for his success in science, in 1848.
Sir Charles Lyell not only ranks as the first of living geologists, but his name will always be closely linked with one of the most important stages in the development of the science. He has been a careful and extensive observer, but he has also eminently a philosophic cast of mind, leading him to the elucidation of principles, and he has accordingly done much to place this great science upon the sound basis of inductive philosophy. Of the extent of his labors in the various special departments of geological observation, it is unnecessary here to speak, but there are one or two great doctrines with the introduction of which into the science of geology his name will be connected, and to which it will be desirable here to refer.
In an able lecture before the Geological Society, in 1869, Prof. Huxley recognized three great systems of geological thought, which he denominates Catastrophism, Uniformitarianism, and Evolutionism. The first phase of thought historically was that of catastrophism. Prof. Huxley defines it as follows: "By catastrophism, I mean any form of geological speculation, which, in order to account for the phenomena of geology, supposes the operation of forces different in their nature, or immeasurably different in power, from those we at present see in action in the universe. The doctrine of violent upheavals and cataclysms in general is catastrophic so far as it assumes that these were brought about by causes which have now no parallel."
This was the earliest system of doctrine in geology, and was inevitable because of the narrow and false ideas concerning past time. Observation accumulated evidences of vast changes in the earth's crust; it was held that the world is but a few thousand years old; it was therefore concluded that the changes must have been on a stupendous scale, of which we have at present no experience.
There are geologists who still hold to this view, as there were those in the early history of the science who believed in vast time and slow changes. Hutton, in his theory of the earth, in 1795, put forth this advanced principle: "I take things such as I find them at present, and from these I reason to that which must have been." It, however, became the work of Sir Charles Lyell to elucidate and establish this doctrine, by extensive and critical investigations, as a broad and fundamental generalization of geological science. Uniformitarianism, or the theory which extends the present rate of terrestrial changes into the past, is a problem of geological dynamics, and involves the study of the totality of forces by which the earth's crust has been altered, and the rocky systems formed.
To work out so vast a subject on the basis of observation and immediate physical data was enough to task the largest capacity, and it is not surprising that Sir Charles Lyell was little disposed to venture into the more speculative questions of the science. Indeed, geologists early insisted on the necessity of limiting inquiry to the changes that have taken place since the formation of the earliest stratified rocks, and crucifying the propensity to pry into the more distant origin of the world. The English Geological Society tacitly forbade these speculations, and of this procedure Mr. Huxley says: "Uniformitarianism, as we have seen, tends to ignore geological speculation in this sense altogether. The one point the catastrophists and the uniformitarianists agreed upon, when this Society was founded, was to ignore it. And you will find, if you look back into our records, that our revered fathers in geology plumed themselves a good deal upon the practical sense and wisdom of this proceeding. As a temporary measure, I do not presume to challenge its wisdom; but in all organized bodies temporary changes are apt to produce permanent effects; and, as time has slipped by, altering all the conditions which may have made such mortification of the scientific flesh desirable, I think the effect of the stream of cold water, which has steadily flowed over geological speculation within these walls, has been of doubtful beneficence."
Mr. Huxley, in common with many other scientists, now holds that the progress of geological thought must carry us beyond uniformitarianism into evolutionism.
But Sir Charles Lyell is the farthest possible from being a narrow-minded partisan. His career offers one of the noblest examples of candor and sincerity, in abandoning old ground and embracing new views, that is afforded in the whole history of science. In the earlier editions of his works he accepted the current opinions in regard to organic species and the past course of life upon earth, but, after a half-century's study of the question, he became satisfied that these views are untenable, and in the tenth edition of his "Principles of Geology," published in 1867, he gave them up, and adopted the general view represented by Mr. Darwin. This great work he has again revised, and the eleventh edition has just made its appearance. All the later questions of this most interesting subject will there be found most ably and fully discussed. We subjoin the notice of the work just published by the Saturday Review:
The great work of Sir Charles Lyell has too long and too authoritatively held its place as a classic in the literature of science to call for, or even to admit, the expression of any estimate of its value. The number of editions it has gone through may be taken as sufficiently attesting the concurrence of public taste and conviction with the appreciative opinion of the more critical class of readers at home and abroad. It may be hoped that the wide and increasing circulation of so valuable a work has had, and is long destined to have, the effect of leavening the mass of educated thought with its sound, careful, and conscientious views of physical truth. While congratulating both the writer and reader upon the issue of the eleventh edition of the 'Principles of Geology,' we feel that our notice of its contents is almost of necessity restricted to those portions of the work in which the author has seen reason to amplify, to remodel, or to correct, what he had advanced in former impressions.
Within the last five years special attention has been drawn to the geological proofs of strongly-marked changes in the terrestrial climate during long periods of time. In face of the additional facts and corresponding theories which have thus divided the minds of geological inquirers, Sir Charles Lyell has seen fit to recast those chapters of his work which treated of the meteorology and climatic history of the earth's surface, with a view especially to insist upon the paramount influence exerted in this direction by the relative distribution and height of the land at successive periods. The balance of argument and research has been such in the mean while as to confirm him more and more in his conviction of the agreement and continuity of the forces at work through all the vicissitudes of the earth's surface, from the earliest to the most recent geological ages. It is hardly necessary, perhaps, to go further back, for the pedigree of the organic forms which for the most part chronicle and attest the laws of succession, than to that Miocene period in whose organic deposits the flora and fauna of all subsequent ages seem to have their ground and root. A superficial view of the local changes of climate which are proved to have taken place might have, and indeed has, induced the belief that causes no longer operative had been at work in remoter times. The existence of a sub-tropical Miocene flora near, and probably up to, the North Pole, with remains of the mastodon, elephant, rhinoceros, and cognate mammals as far north as the icy circle, might be taken to point to a revolution of a terrestrial, if not of a cosmical, kind from higher conditions of temperature. On the other hand, the dispersion in a southern direction of erratic blocks, evidently carried by ice-action, and striated or polished by glacial friction, was a proof of a cold climate extending much farther south than that of the present time, invading. even the subtropical latitudes. Now, there can be absolutely no room for the hypothesis of any appreciable change, within Miocene times at least, in the total temperature of the earth, either from the sudden outburst of subterranean fires on the one hand, or from general cooling of the earth's mass on the other. At the same time, a large body of both organic and inorganic evidence supports the view that the climate of earlier geological periods, from whatever cause, had over wide regions been in excess of what it now is. Not only in the greater part of the Miocene and Eocene epochs did a vegetation like that of Central Europe in our day extend into the Arctic regions as far as they have been explored, and probably to the Pole itself, but in the Secondary or Mesozoic ages the prevalent types of vertebrate life indicate a warm climate and an absence of frost between latitude 40° north and the Pole, a large ichthyosaurus having been found in latitude 77° 10' north. Carrying our retrospect back to the Primary or Palæozoic ages, we find an assemblage of plants which implies that a warm, humid, and equable climate extended from the 30th parallel of north latitude to within a few degrees of the Pole, while a still older flora, the Devonian, leads to a similar inference. Such, moreover, is the general resemblance between the whole invertebrate fauna of the Devonian, Silurian, and Cambrian rocks and that of the Carboniferous, Permian, and Triassic series, as to make it clear that a similarity of conditions as regards temperature prevailed throughout the whole of these six periods.
The idea of possible variations in the temperature of space traversed by our globe, started by Poisson, is promptly set aside by considerations long ago advanced by Mr. Hopkins. Nor is there much greater force, as Sir Charles Lyell amply shows, in the effect attributed by others to variation in the obliquity of the ecliptic. The latest calculations of Sir John Herschel conveyed in a letter to our author, in October, 1866, admit the possibility of a deviation of the earth's axis to the extent of three, or even four, degrees on either side of the mean. The sun's rays would thus be disseminated at intervals over a far broader zone than at present, around the Arctic and Antarctic Poles, with a corresponding shortening of the Polar night, and a diffusion of more genial warmth. Yet, on the other hand, a large deduction must be made, as Mr. Meech has shown, for the increased length of path, and the greater amount of atmosphere through which the calorific rays must pass in very high latitudes, not to speak of the greater prevalence of cloud in regions round the Pole. A truer cause of climatic change Is to he sought in the effect of precession of the equinoxes, the revolution of the apsides, and, above all, the eccentricity of the earth's orbit. The great cycle of change due to precession would cause the different seasons of the Northern and Southern Hemispheres to coincide in turn, within 25,868 years, with all the points through which the earth passes in its. orbit round the sun. Combining with this movement, that of the revolution of the apsides or "motion of the aphelion," as Herschel named it, reduces this term of years to about twenty-one thousand. Sir C. Lyell's explanation, aided by a new diagram, renders sufficiently clear the effects which would be produced upon climate by the successive phases of precession, especially when combined with increased eccentricity or distance from the sun. The difference between winter in aphelion and perihelion—the range of eccentricity extending, as he has shown, to 14,000,000 miles at some periods, instead of 3,000,000, as now—is set down by Mr. Croll as not less than one-fifth of the entire heat received from the sun. Some slight change in this direction since the year 1248 A.D. has been thought capable of actual proof by M. d'Adhémar, and of being verified by the observations of M. Venetz upon the decrease of Swiss glaciers prior to the tenth century, and their subsequent increase. An admirable table compiled by Mr. Stone shows the variations in eccentricity for a million years before 1800 A.D., with the number of days which would be added to winter by its occurrence in aphelion, which has been followed up for a million years more by Mr. Croll and Mr. Carrick Moore. From these figures there might appear to be a possibility of approximating to a date for the Glacial epoch; and Sir C. Lyell holds it "far from startling" that 200,000 years back might be fixed upon as about the period of greatest cold, when the excess of winter days amounted to 27.7. He had in his tenth edition speculated upon 800,000 or 1,000,000 years as nearer the Glacial epoch, but he feels compelled to narrow the time within the limit at which the principal geographical features of the continents and oceanic basins were approximately assuming their present form. Were the astronomical theory, however, to be relied upon as the basis for the solution of the problem, we ought to meet in the course of palæontological research with a series of Glacial periods perpetually recurring in the Northern Temperate Zone; supposing a large eccentricity by itself sufficient apart from the cooperation of terrestrial causes, to intensify the cold of high latitudes. But no such evidence of violent revolutions is to be found in the flora and fauna of earlier periods. The continuity of forms, particularly in the class of reptiles, from the Carboniferous to the Cretaceous period, is an obvious fact opposed to the intercalation of intense glacial epochs. Another fact is, that many great cycles of eccentricity must have been gone through in the long centuries of the Carboniferous period, in which no break in the order of life is manifested.
The exhaustion of all other means of solution, joined to the mass of positive evidence accumulated by recent science, throws us more and more conclusively upon the idea to which Sir Charles Lyell has firmly held from the first, and which may be taken as the culminating point of his latest achievements in geology, that the predominant cause of the great changes in climate is to be found in the distribution and elevation of the land. The Glacial period may be traced to an excessive and abnormal accumulation of land around the Pole. There is absolutely no limit to the alternations which the surface of our globe may have, or indeed has gone through. There is hardly a spot of what is now land which has not been covered by the sea, probably not a space now covered by the ocean which has not been at some time, if not many times, dry land. In one epoch the land may have been chiefly equatorial, at another polar or circumpolar. At present we may readily divide the globe into two equal parts, the land hemisphere and the water hemisphere; the former of which exhibits almost as much land as water, or as 1 to 1.106; while in the latter the proportion of land to water, as made out by Mr. Trelawny Saunders, is only as 1 to 7.988. The general proportion of land to sea may be taken throughout the globe as 1 to 2½. Were the land, by the action of subterranean forces, its total amount being unchanged, now gathered together in masses along the equator and around the Poles alternately, such geographical changes would amply suffice, as Sir C. Lyell makes it his task to show, to explain the utmost vicissitudes which the climate of the earth has undergone. This course of reasoning by no means precludes such aid as may be brought in by independent verce causæ, by the concurrence of the cold period induced by excessive piling of land around the Pole with wintering in aphelion, or at a period when the earth's axis was abnormally inclined. These causes, especially in combination, would greatly intensify what, after all, must remain the ruling and inherent principle of climatic revolutions. We have only to look at the present aspect of Greenland to satisfy ourselves what might become the state of the British Isles by a mere substitution of other local conditions under the same parallel of latitude. Were the Gulf Stream done away with, the equatorial continents which now form vast reservoirs of heat transferred to the Northern regions, and their snow-clad frozen surface swept by Polar currents, how far south would the ice-sheet cover the unsubmerged tracts of land, and the glaciers come down to the level of the sea? The chain of facts and reasonings by which Sir Charles Lyell binds together the phenomena which science and discovery contribute to this intricate problem forms one of the most characteristic features of his book. Every new link, and every additional degree of tenacity given to his argument, enhances the value of this standard work as a steadfast, clear-sighted, and consistent witness to the great law of uniformity and continuity in Nature.
The latest information acquired by deep-sea dredging has been incorporated by Sir C. Lyell into his remarks upon the temperature and shape of the bed of the ocean and its living inhabitants. In his chapter on ocean-currents he has also considered the latest-known results of experiments and observations made by Dr. Carpenter, Prof. Wyville Thompson, and Captains Spratt and Nares, upon the currents of the Straits of Gibraltar. The space allotted to this survey is not adequate to a full or critical discussion of the arguments for and against the existence of a permanent indraught. The balance of proof, however, is felt by Sir Charles to support his previously-expressed conviction that the inflowing movement is no permanent undercurrent caused by evaporation, but the result of the Mediterranean tide, which, slight as it is, runs alternately from east to west for several hours, its action being found more regular in the depths of the straits, where it is less affected either by winds or by the surface inflow. The difference of no less than twenty degrees between the temperature of the Mediterranean and the Atlantic, as well as the difference of four degrees between the deep-sea soundings of the western and central basins of the Mediterranean and of the Greek Archipelago, is explained by the existence of high submarine crests or barriers of rock bounding the sea to the west, and again dividing it into sections, as shown by the diagram in the present edition. Proceeding to the wider problem of ocean circulation arising out of the extreme cold found at great depths both in temperate and tropical regions, Sir Charles disputes the notion of these low temperatures being due to mere depth, the Mediterranean soundings of 13,800 feet having failed to reach a degree of cold below 55° Fahr. Yet the soundings taken at Aden, whither the cold water can only come from the Southern Hemisphere, lead to the belief that the whole of the equatorial abysses of the ocean are traversed, in some parts at least, by a continuous mass of water not much above 32° Fahr. That solar heat is in some way or other the primary cause of this great displacement, through the change in specific gravity from the cooling of water toward the polar zones, counterbalanced by a return, however slowly, of water from the equator to the Poles, may well take the place of more recondite theories, such as that exploded by Herschel, that the expansion of water by heat in the equatorial zone raises the level of the sea, and causes a flow down a gently-inclined plane toward the Poles. In the absence, however, of more extensive and accurate knowledge of the state of the ocean at great depths, or of its local direction and quantity of motion, in relation to the utter stillness found generally by the sounding-line to prevail in its great abysses, Sir Charles Lyell is too cautious and patient a reasoner to think the time ripe for a positive solution.