Geological time scale

“ The belief that the mathematical physicist must be right in his views as to the age of the sun’s heat, and that there is no possibility of a longer period being admitted, seems at present to be leading geologists towards the opposite extreme in regard to the length of geological time. Attempts have been recently made to compress the geological history of our globe into the narrow space allotted by the physicist. ”

“ Can you measure either of those intervals in the years of common chronology? Yet all over the world the succession of geological strata proclaims the recurrence over and over again of such intervals; silent, indeed, as to positive evidence, but widening the possible limits of time’s duration to the furthest stretch of fancy.
All our great continents have been ever so many times, either in the mass or piecemeal, under the waves of the ocean. Nothing hinders that the bed of every great ocean should have been ever so many times turned into dry land. ”

“ Whether we seek for astronomical or geological data, we find great uncertainty. To such an extent is this the case, that current estimates of the time necessary to bring the earth from a state of primitive incandescence to its present condition have varied from fifteen millions of years to five hundred millions. ”

“ For certainly on my theory such strata must somewhere have been deposited at these ancient and utterly unknown epochs in the world's history.
I can answer these questions and grave objections only on the supposition that the geological record is far more imperfect than most geologists believe. It cannot be objected that there has not been time sufficient for any amount of organic change; for the lapse of time has been so great as to be utterly inappreciable by the human intellect. ”

“ Though this epoch is incredibly more remote than any historical record, it is perhaps not incomparable with the duration of the human race; while compared with the vast lapse of geological time, such periods seem trivial and insignificant. Geologists have long ago repudiated mere thousands of years; they now claim millions, and many millions of years, for the performance of geological phenomena. ”

“ It is impossible to give any trustworthy estimate of the actual lengths of these divisions in years, though many attempts to do so have been made. All that can be confidently affirmed is that geological time, like astronomical distances, is of inconceivable vastness and its years can be counted only in hundreds of millions. ”

“ The fossil remains of the Tertiary rocks, both animal and vegetable, present us with many instances of this kind.
The discussion of the distribution of animals and plants, when carried on in the light of geology, raises many interesting questions as to time, which we have already glanced at, but which deserve a little more attention. As to the vast duration of geological time all geologists are agreed. It is, however, now well understood that science sets certain limits to the time at our disposal. ”

“ Professor Tait probably over-estimates the time when he affirms “that 10,000,000 years is about the utmost that can be allowed, from the physical point of view, for all the changes that have taken place on the earth’s surface since vegetable life of the lowest known form was capable of existing there.” ”

“ But, although we may be unable to estimate the minimum of time required for the changes in physical geography above alluded to, we cannot fail to perceive that the duration of the period must have been very protracted, and that other ages of comparative inaction may have followed, separating the post-pliocene from the historical periods, and constituting an interval no less indefinite in its duration. ”

“ In this discovery of organic remains in the primary rocks, we hail with joy the dawn of a new epoch in the critical history of these earlier formations. Already in its light, the primeval geological time is seen to be everywhere animated, and peopled with new animal forms of whose very existence we had previously no suspicion. Life, which had hitherto been supposed to have first appeared in the Primordial division of the Silurian period, is now seen to be immeasurably lengthened beyond its former limit, and to embrace in its domain the most ancient known portions of the earth's crust. ”

“ Date of Last Glacial Epoch and its Bearing on the Measurement of Geological Time.—Directly we go back from this stable period we come upon changes both in the forms and in the distribution of species; and when we pass beyond the last glacial epoch into the Pliocene period we find ourselves in a comparatively new world, surrounded by a considerable number of species altogether different from any which now exist, together with many others which, though still living, now inhabit distant regions. ”

“ He next takes up the estimates of geologists, and rather blames as too modest those who ask for the longest time, say 400 millions of years, for the duration of the habitable earth. He evidently scarcely deems worthy of notice the more moderate demands of many eminent students of the earth, who have based far lower estimates on more or less reliable data of denudation and deposition, and on the thickness of deposits in connection with their probable geographical extent. ”

“ The duration of each of the phases of Croll's theory corresponds also, on the whole, remarkably well with that required for each phase of the geological record. They would average about 40,000 years each for Croll's phases, and a less time can hardly be allowed for the immense amount of geological work in the way of 316 denudation and deposition, elevation and depression, and changes of fauna and flora which have occurred since the commencement of the great refrigeration in the late Pliocene. ”

“ Hence, trifling as the results during a human life may appear, if we realize the multiplying influence of time we are led to perceive that the apparently feeble superficial agents can, in the course of ages, achieve stupendous transformations in the aspect of the land. If this efficacy may be deduced from what can be seen to be in progress now, it may not less convincingly be shown, from the nature of the sedimentary rocks of the earth’s crust, to have been in progress from the early beginnings of geological history. ”

“ A lasting service will be rendered to our science if by well-concerted effort we can place geological dynamics and geological chronology on a broader and firmer basis of actual experiment and measurement than has yet been laid.
To understand aright the origin and progress of the dispute regarding the value of time in geological speculation, we must take note of the attitude maintained towards this subject by some of the early fathers of [Pg 200] the science. Among these pioneers none has left his mark more deeply graven on the foundations of modern geology than James Hutton. ”

“ It is evident that changes of climate such as are shown to have occurred must have arisen from general, not from any local cause or accidental combination of causes, and if this be so there may yet appear a reliable means of determining not only the amount and extent of the changes, but the periods of their duration and recurrence. The question of time in geological history is an important and certainly a most interesting one, and the interest in it has increased since the announcement by Dr. ”

“ We may observe, however, that the recent researches, which have contributed so much to fill up the interstices of the Geological Calendar, have conduced in no small degree to fill up likewise some of the more remarkable gaps or chasms in the succession of Organic Life. It is, therefore, not unreasonable to suppose that, as our knowledge of the Earth’s Crust becomes more and more minute, the sudden breaks in the continuity of the scale 223 will be still further diminished and the successive stages of gradual transition will be made more clearly apparent. ”

“ Astronomers deal with distances so great that they are almost beyond understanding; nuclear physicists study objects so small that we can hardly imagine them. Similarly, the geologist is concerned with spans of time so immense that they are scarcely comprehensible. Geology is a science of time as well as rocks, and in our geologic story of the Teton region we must refer frequently to the geologic time scale, the yardstick by which we measure the vast reaches of time in earth history. ”

“ Now I would ask geologists whether it is not dangerous to strive after chronological precision in geological inquiries by reasoning which assumes that nature worked during a long period of remote time at a uniform rate of speed. The calculations by which Sir Archibald Geikie laboured years ago to estimate the time which the Thames occupied in excavating its valley,2370 the calculations which geologists have made as to the time required for the deposition of the layers of stalagmite in caves,2371 have been proved to be futile. ”

“ We can be far more certain of the time which must have been required to remove by denudation, say, a thousand feet of rock than we can possibly be of the time required to have deposited a thousand feet of sediment. The thousand feet of sediment may, under certain conditions, have been deposited in a hundred years, while under other conditions they may have required a million of years. In fact, nothing can be more uncertain than the rate of deposition: it depends upon such a multitude of circumstances. ”

“ Rocks carrying remarkably similar fossils may really be several thousand years different in age; but this is, indeed, a very small limit of error when one considers the vast antiquity of the earth. Much very accurate and satisfactory work has been done, especially in Europe and North America, in correlating strata and assigning them to their places in the geological time table (see below) , but a vast amount of work yet remains to be done before the task is complete.
Certain types or species of organisms are much more useful than others in the determination of earth chronology. ”

“ By methods based on the approximate uniformity of denudative
effects in the past, a period of the order of 100 millions of
years has been obtained as the duration of our geological age;
and consistently whether we accept for measurement the sediments
or the dissolved sodium. We can give reasons why these
measurements might afford too great an age, but we can find
absolutely no good reason why they should give one much too low. ”

“ No living species goes back farther than the Pliocene. Few extend farther than the Glacial age. On the same principle it is not to be expected that man, the highest of all animals, should extend far back in geological time.
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Accepting the Post-glacial age as that of the advent of man, it may be interesting to ask what we know of the condition of our continents when he appeared. In Western Asia, in Europe, except in its more northern portions, and it would now seem also in America, man had been introduced at a time closely following the emergence of the land from the Glacial sea. ”

“ I have used it, however, for want of a better, and as an expression of convenience, always intending to express by the phrase "Glacial epoch" that section of geological time which was typically distinguished by the
prevalence of severe climatal conditions through a great part of the northern hemisphere, and during which those marine accumulations, in part truly sedimentary deposits, which have been called "Northern drift," were formed. ”

“ It has often been objected that the present rate of geological change ought not to be taken as a measure of the rate in past time, because the total sum of terrestrial energy has been steadily diminishing, and geological processes must consequently have been more vigorous in former ages than they are now. Geologists do not pretend to assert that there has been no variation or diminution in the activities of the various processes which they have to study. ”

“ Fossils provide important clues to the ages of the rocks in which they are found. The slow evolution of living things through geologic time can be traced by a systematic study of fossils. The fossils are then used to determine the relative ages of the rocks that contain them and to establish a geologic time scale that can be applied to fossil-bearing rocks throughout the world. Figure 18 shows the major subdivisions of the last 600 million years of geologic time and some forms of life that dominated the scene during each of these intervals. ”

“ Fig. 3.—Relative Length of Ages of Reptiles, Mammals and Man.
Length of Geologic Cycles. But these vast cycles of geographic and climatic change will take millions of years to accomplish their course. The brief span of human life, or even the few centuries of recorded civilization are far too short to show any perceptible change in climate due to this cause. The utmost stretch of a [20] man's life will cover perhaps one-two hundred thousandth part of a geologic period. The time elapsed since the dawn of civilization is less than a three-thousandth part. ”

“ It cannot be too frequently stated, nor too prominently kept in view, that, although gaps occur in the succession of organic remains as recorded in the rocks, they do not warrant the conclusion that any such blank intervals ever interrupted the progress of plant and animal life upon the globe. There is every reason to believe that the march of life has been unbroken, onward and upward. Geological history, therefore, if its records in the stratified formations were perfect, ought to show a blending and gradation of epoch with epoch. ”