OCR Text |
Show 216 ISLAND LIFE. (PART I. present rate of denudation and deposition, is only 28,000,000 years.1 The Rate of Geological Change pTobably ,qreater in very remote times.-The opinion that denudation and deposition went on more rapidly in early times owing to the frequent occurrence of vast convulsions and cataclysms was strenuously opposed by Sir Charles Lyell, who so well showed that causes of the very same nature as those now in action were sufficient to account for all the phenomena presented by the rocks throughout the whole series of geological formations. But while upholding the soundness of the views of the "uniformitarians " as opposed to the "convulsionists," we must yet admit that there is reason for believing in a gradually increasing intensity of all telluric action as we go back into past time. This subject has been well treated by Mr. W. J. Sollas, 2 who shows that, if, as all physicists maintain, the sun gave out perceptibly more heat in past ages than now, this alone would cause an increase in almost all the forces that have brought about geological phenomena. With greater heat there would be a more extensive aqueous atmosphere, and a greater difference between equatorial and polar temperatures ; hence more violent winds, heavier rains and snows, and more powerful oceanic currents, all producing more rapid denudation. At the same time, the internal heat of the earth being greater, it would be cooling more rapidly, and thus the forces of contraction-which cause the upheaving of mountains, the eruption of volcanoes, and the subsidence of extensive areas-would be more powerful and would still further aid the process of denudation. Yet again, the earth's rotation was certainly more rapid in very remote times, and this would cause more impetuous tides and still further add to the denudin<Y 0 1 From the same data Professor Haughton estimates a minimum of 200 million years for the duration of geological time · but he arrives at this conclusion by supposing the products of denudation to be uniformly spread over the whole sea-bottom instead of over a narrow belt near the coasts, a suppouition entirely opposed to all the known facts, and which had been shown by Dr. Croll, five years previously, to be altogether erroneous.. (S.ee Nature, Vol. XVIII., p. 268, where Professor Haughton's paper 1s giVen as read before the Ho)"Ul Society.) ~ See Geological Magazine for 1877, p. 1. · CHAP. X.] 'rHE EARTH'S AGE. 217 power of the ocean. It thus appears that, as we go back into the past, all the forces tending to the continued destruction and renewal of the earth's surface ·would be in more powerful action, and must therefore tend to reduce the time required for the deposition and upheaval of the various geological formations. It may be true, as many geologists assert, that the changes here indicated are so slow that they would produce comparatively little effect within the time occupied by the known sedimentary rocks, yet, whatever effect they did produce would certainly be in the direction here indicated, and as several causes are acting together, their combined effect may have been by no means unimportant. It must also be remembered that such an increase of the primary forces on which all geologic change depends would act with great effect in still further intensifying those alternations of cold and warm periods in each hemisphere, or, more frequently, of excessive and equable seasons, which have been shown to be the result of astronomical, combined with geographical, revolutions; and this would again increase the rapidity of denudation and deposition, and thus still further reduce the time required for the production of the known sedimentary rocks. It is evident therefore that these various considerations all combine to prove that, in supposing that the rate of denudation has been on the average only what it is now, we are almost certainly over-estimating the time required to have produced the whole series of formations from the Cambrian upwards. Val~te of the preceding estimate of Geological Time.-It is not of course supposed that the calculation here given makes any approach to accuracy, but it is believed that it does indicate the order of magnitude of the time required. We have a certain number of data, which are not guessed but the result of actual measurement; such are, the amount of solid matter carried down by rivers, the width of the belt within which this matter is mainly deposited, and the maximum thickness of the known stratified rocks.1 .A considerable but unknown amount of 1 In his reply to Sir W. Thompson, Professor Huxley assum,ed one foot in a thousand years as a not improbable rate of deposition. The above estimate indicates a far higher rate ; and this follows from the well |