Geological time

“ 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. ”

“ As to the cosmical nature of the periods, while some geologists appear to regard the whole of geological time as a continuous evolution without any breaks, it is evidently more in accordance with facts to hold that there have been cycles of repose and activity succeeding each other, and that these have been of different grades. ”

“ 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. ”

“ The terms used for the more important divisions of geological time are, in descending order of magnitude, era, period, epoch, age or stage, and the general scheme of the eras and periods, which is in almost uniform use throughout the world, is given in the table, which is arranged so as to give the succession graphically, with the most ancient rocks at the bottom and the latest at the top. ”

“ One of the first questions meeting us is that which relates to the point in geological time signalised by the advent of our species. In the Eocene period our continents were being gradually raised out of the ocean, and were still in great part under the waters, which several times returned upon the land, and seemed ready again to engulf it. In this period not only have we no traces of man, but all the higher animals of that age are now extinct. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ If we knew the total quantity of stratified rock on the globe, we could easily tell the time that would be required for its formation. Most geologists would, I believe, be inclined to admit that, if spread uniformly over the entire globe, it would form a layer of at least 1,000 feet in thickness. In such a case the time required for its deposition would be as follows:
1,000 × 6,000 × 4 = 24,000,000 years. ”

“ 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. ”

“ By the study of the successive beds, more especially of those deposited in the times of continental submergence, we obtain a table of geological chronology which expresses the several stages of the formation of the earth’s crust, from that early time when a solid shell first formed on our nascent planet to the present day. By collecting the fossil remains embedded in the several layers and placing these in chronological order, we obtain in like manner histories of animal and plant life parallel to the physical changes indicated by the beds themselves. ”

“ In studying the various geological agencies we must remember the almost inconceivable times in which they work. The slowest process when multiplied by the immense time in which it is carried on produces great results. The geologist looks upon the land forms of the earth’s surface as monuments which record the slow action of weathering and other agents during the ages of the past. ”

“ If we reflect on the long series of events of the post-pliocene and recent periods contemplated in this chapter, it will be remarked that the time assigned to the first appearance of man, so far as our geological inquiries have yet gone, is extremely modern in relation to the age of the existing fauna and flora, or even to the time when most of the living species of animals and plants attained their actual geographical distribution. ”

“ Geological terms convey to us no definite ideas as to the lapse of time, and we have an instinctive desire to substitute for them some term of years. In most cases this is impossible, as we have no means to measure the flight of past time, nor are we yet prepared to discuss the question of time, since to do so we must learn a great deal more about the cause of the Glacial Age. ”

“ Yet that time must have been a fact not less possible of verification than that geological period when “the earth was absorbed in producing the carboniferous vegetation which still supplies us with the means of warmth, light, and life, accumulating during enormous periods of time small deposits of organic matter forming the strata of the globe on which we live. ”

“ The arguments from the geological side against the physical contention that would limit the age of our globe to some 10 or 20 millions of years are mainly based on the observed rates of geological and biological changes at the present time upon land and sea, and on the nature, physical history and organic contents of the stratified crust of the earth. ”

“ And of this the whole science of geology affords the demonstration. If the rocks have been formed in succession, as geology has proved, and twenty miles of strata have been piled over the earliest-appearing forms of life, then the time since living creatures came has been indefinitely vast, and that the periods are not amenable to anything like "calculation" or trustworthy estimate is shown by the way the subject is dealt with in our most authoritative geological works. ”

“ Computation of the Average Annual Amount of Subaërial Denudation.—The geology of the district above alluded to may assist our imagination in conceiving the extent to which groups of ancient rocks, each of which may in their turn have formed continents and oceanic basins, have been disturbed, folded, and denuded even in the course of a few out of many of those geological periods to which our imperfect records relate. It is not easy for us to overestimate the effects which causes in every day action must produce when the multiplying power of time is taken into account. ”

“ I did not, at the time, fully appreciate the deep conviction which it displays of the slow manner in which geological changes have taken place, and the insignificance of thirty or forty centuries in the history of a species, and that, too, at a period when very narrow views were entertained of the extent of past time by most of the ablest geologists, and when great revolutions of the earth's crust, and its inhabitants, were generally attributed to sudden and violent catastrophes. ”

“ The conclusion is, on the whole, that the periods allowed by physicists are not only far in excess of such as are required for geological and organic change, but that they allow ample margin for a lapse of time anterior to the deposit of the earliest fossiliferous rocks several times longer than the time which has elapsed since their deposit to the present day. ”

“ So long as physiologists continued to believe that man had not existed on the earth above six thousand years, they might, with good reason, withhold their assent from the doctrine of a unity of origin of so many distinct races; but the difficulty becomes less and less, exactly in proportion as we enlarge our ideas of the lapse of time during which different communities may have spread slowly, and become isolated, each exposed for ages to a peculiar set of conditions, whether of temperature, or food, or danger, or ways of living. ”

“ But both ends of the geological scale, comprising those ancient systems older than the Coal, and representative of periods in which, so far as is yet known, life, animal and vegetable, first began upon our planet, and those systems of comparatively modern date, representative of the periods which immediately preceded the human epoch, were equally unknown. ”

“ Every rock beneath our feet has its story of change to tell us. Mountains, valleys, and plains, continents and islands, have passed through vicissitudes innumerable, and bear within them the evidence of a gradual development or evolution. Looking back through the vista of the past one sees the dry lands gradually separating from the ocean, and gathering together into continental masses according to a definite plan. It is this slow growth, this august evolution, carried on through countless æons, which most impresses the student of physical geology. ”

“ We shall see hereafter how the relative position of these two kinds of rocks and their action upon each other enables us to determine the chronology of the earth, to compare the age of her mountains, and if we have no standard by which to estimate the positive duration of her continents, to say at least which was the first-born among them, and how their characteristic features have been successively worked out. I am aware that many of these inferences, drawn from what is called "the geological record," must seem to be the work of the imagination. ”

“ 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. ”

“ The conditions of life must have been greatly modified by these geographical changes. The life-forms bear testimony to this changed condition. Old forms die away, and are succeeded by those approaching more nearly our own times. The name of this period is the Mesozoic time, or the period of middle life forms.14 It is instructive to notice the steady advance in the type of life, both animal and vegetable. The abundant flowerless vegetation of the coal formation of the preceding epoch dwindles away. ”

“ From the time of Linnæus to the commencement of the present century, it seemed a sufficient definition of the term species to say, that 'a species consisted of individuals all resembling each other, and reproducing their like by generation.' But Lamarck, after having first studied botany with success, had then turned his attention to conchology, and soon became aware that in the newer (or tertiary) ”

“ This vast period of time, which includes the beginning, is known among geologists as Archean time.
From the close of this age, the history of life properly commences. It might be well to explain the means which the geologist uses to interpret the history of the globe. It is now understood that the forces of nature have always produced the same results as they do now. From the very earliest time to the present, rocks have been forming. ”

“ In the whole field of natural science there is no more noble problem. Take, for examine, that earliest and most interesting epoch when life perhaps commenced on the earth, and when stratified rocks were deposited five or ten miles thick, which seem to have contained no living forms higher than the humble Eozoön, if even that were an organized being. Let us ask what the distance of the moon was at the time when those stupendous beds of sediment were deposited in the primeval ocean. ”

“ With regard, however, to the existing terrestrial order, a great deal can be discovered, and to do so is one of the principal tasks of geological science. The first steps in the production of that order lie buried in the profound and unsearchable depths of a past so prolonged as to present itself to our finite minds as almost in eternity. ”