Geochronology

“ In the first place, we fully admit that the extreme 283 Antiquity of the Earth is a necessary consequence of our theory. Setting out from the present stage of the world’s existence, Geology carries us back from epoch to epoch, through a long succession of ages, each extending over many thousand years, until the mind is lost in the seeming infinity of the past. It may be asked, perhaps, in what way Geology can testify to the great length of each successive period in the history of the Globe. ”

“ The first or palæolithic epoch is separated from the second or recent epoch by a vast and unknown lapse of time. We may place its date at somewhere about 200,000 years back. The remains of human origin belonging to it all occur under the conditions which we ordinarily describe as geological; they are found either in the drift deposits of our river-valleys or beneath the concreted floors of caves. They consist chiefly of rude stone weapons, in unpolished flint, chipped off by side-blows. ”

“ Most people who have not specially studied prehistoric archæology, and many of those who have studied it, believe that the period of human life on the earth may be divided into three principal epochs, the Iron age, the Bronze age, and the Stone age; and that the last named epoch may be once more subdivided into the Palæolithic and the Neolithic ages. All the great archæologists know, of course, that such a division would be utterly misleading; yet, in their written works, they have often used language which has led the world generally to fall, almost without exception, into the error. ”

“ 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. ”

“ Counting back gives an absolute age for the artifacts embedded in the site and thus an approximate age for the site. The European varve chronology is believed to extend back to about 9650 B.C.
Most of the clocks which the archaeologist uses to produce relative dates, the before-or-after kind, have as their theoretical basis the principle of stratigraphy. In effect the principle of stratigraphy assumes two things: that the rocks of the earth are constantly wearing down by erosion, and that things which appear to be alike actually are alike and are probably more or less the same age. ”

“ The only value of such estimates is to define our notions of geological time, and to show that the enormous periods, of hundreds of millions of years, which have sometimes been indicated by geologists, are neither necessary nor warranted by the facts at our command; while the present result places us more in harmony with the calculations of physicists, by leaving a very wide margin between geological time as defined by the fossiliferous rocks, and that far more extensive period which includes all possibility of life upon the earth. ”

“ The earlier Post-pliocene [283] period of geology may be called the Glacial era. It is that of a cold climate, accompanied by glaciation and boulder deposits. (2) The later Post-pliocene may be called the Post-glacial era. It is that of re-elevation of the continents and restoration of a mild temperature. It connects itself with the pre-historic period of the archæologist, inasmuch as remains of man and his works are apparently included in the same deposits which hold the bones of Post-glacial animals. (3) The Modern era is that of secular human history. ”

“ If the interpretation of the text be correct, calculation would fix the date between the year 3,007 and 3,010 of the ancient era, or a thousand years after the epoch attributed to the reign of Menkera in Mariette's chronological table. In any case, it is to be hoped that future discoveries will enable us with certainty to trace back the course of ages, and to determine positive dates for the origins of history, with which may be connected the fluctuating chronology of the most remote events of which the human race has preserved the memory. ”

“ When we study, therefore, the geological records of the earth and its inhabitants, we find, as in human history, the defectiveness and obscurity of the monuments always increasing the remoter the era to which we refer, and the difficulty of determining the true chronological relations of rocks is more and more enhanced, especially when we are comparing those which were formed simultaneously in very distant regions of the globe. ”

“ 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. ”

“ These three principal phenomena of the ice-fields, the mammals, and the human life and industry, together establish the chronology of the Age of Man. In other words, there are four ways of keeping prehistoric time: that of geology, that of palæontology, that of anatomy, and that of human industry. Geologic events mark the grander divisions of time; palæontologic and anatomic events mark the lesser divisions; while the successive phases of human industry mark the least divisions. ”

“ If the geologist did not tell us, how should we ever suspect that probably where we now stand two or more miles of strata have been worn away by the winds and rains; that the soil of our garden, our farm, represents the ashes of mountains burned up in the slow fires of the geologic ages.
Geology first gives us an adequate conception of time. The limitations which shut our fathers into the narrow close of six thousand years are taken down by this great science and we are turned out into the open of unnumbered millions of 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. ”

“ 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. ”

“ At present we have a tree ring calendar for certain species of trees extending back to about 100 B.C. This is called a master tree ring chronology. A log from an archaeological site may contain only one or two signatures and be, in itself, a floating chronology, but by comparison with the master chronology one can determine the cutting date. Having a series of cutting dates for the construction timbers in an archaeological site yields the probable dates at which the site was built and occupied. ”

“ It will be necessary before we examine the group of caves used by man in prehistoric times, to point out the important difference in the measurement of time within and beyond the borders of history. When we speak, for example, of the date of the Norman Conquest, we imply that we can ascertain by historical records, not merely that it succeeded the invasion of Britain by the English or Danes, and happened before our own time, but that the interval which separates it from those events can be accurately measured by the unit of years. ”

“ But we have not yet done with this part of our investigation. The argument from geology is based on the assumption that the chronological order of the earth's layers has been determined at least with great approximation to certainty, so that we may say with some assurance that this layer of limestone or sandstone is of earlier, that, of later origin. As a matter of fact, the textbooks do treat the various "ages" of geology as if they corresponded to certain strata of the earth's crust. But by what method is the age of the various layers determined? ”

“ While the nature and mode of formation of the rocks may thus be postulated, it will be needful to explain at some length the character of the evidence from which the history of the earth may be deciphered. First of all, must be made clear the method by which the events of the earth’s history may be arranged in chronological order, for a history without chronology is unintelligible. The events which are most significant for our purpose are recorded in the rocks which are called stratified, bedded or sedimentary, synonymous terms. ”

“ Modern geology and archæology have rendered valuable assistance toward its solution, and at the present stage of these sciences it seems most probable that the time of man's advent must be placed in the tertiary or in the beginning of the diluvial age. ”

“ In the literature of the last century, we find frequent allusion to the 'era of existing continents,' a period supposed to have coincided in date with the first appearance of man upon the earth, since which event it was imagined that the relative level of the sea and land had remained stationary, no important geographical changes having occurred, except some slight additions to the deltas of rivers, or the loss of narrow strips of land where the sea had encroached upon its shores. ”

“ We must get the series of formations in each case, and compare them with each other, see whether the changes in the one answer to those in the other, and endeavour to trace out some common starting-point of time, before we shall be able to draw clear geological horizons, establish definite chronological epochs common to the whole earth. ”

“ In thus inverting what might at first seem to be the more natural order of historical research, we must bear in mind that each of the periods above enumerated, even the shortest, such as the Post-tertiary, or the Pliocene, Miocene, or Eocene, embrace a succession of events of vast extent, so that to give a satisfactory account of what we already know of any one of them would require many volumes. ”

“ 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. ”

“ Of course the periods and eras into which the geologists divide geologic time are as arbitrary as the months and seasons into which we divide our year, and they fade out into each other in much the same way ”

“ The Saros of the Chaldaeans, the Olympiad of the Greeks, and the Roman Indiction are instances of this mode of reckoning time. Several cycles were formerly known in Europe; but most of them were invented for the purpose of adjusting the solar and lunar divisions of time, and were rather employed in the regulation of the calendar than as chronological eras. They are frequently, however, of very great use in fixing dates that have been otherwise imperfectly expressed, and consequently form important elements of chronology. (W. L. R. C.)
Modern Results of Archaeological Research. ”

“ Geologists call the last great geological period the Pleistocene. It began somewhere between a half million and a million years ago, and was a time of great changes. Sometimes we call it the Ice Age, for in the Pleistocene there were at least three or four times when large areas of earth were covered with glaciers. The reason for my uncertainty is that while there seem to have been four major mountain or alpine phases of glaciation, there may only have been three general continental phases in the Old World.2 ”

“ 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. ”

“ 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. ”