GEOLOGICAL TIME. 23 



shingle, or limestone had actually to be formed ; whereas in 

 each succeeding period the fresh deposits were many of them 

 largely made up of the older sedimentary strata ; and there- 

 fore new deposits may be laid down more rapidly at the 

 present day than could have been the case under similar 

 conditions in former times. Mr. T. Mellard Reade has dis- 

 cussed the question of Time from this point of view, dealing 

 with the denudation of the land, which can be estimated, and 

 then calculating the rate at which various rocks are destroyed, 

 and the rate at which material carried away from the land in 

 solution and suspension may be again accumulated. The 

 results, although interesting and suggestive, do not enable us to 

 estimate with accuracy the duration of our geological periods. 



Calculations, such as that by which the formation of the 

 Coal-field of South Wales has been estimated at 640,000 

 years, are, however, not without their use in giving a rough 

 notion of the time occupied in the accumulation of strata ; for 

 while we know the relative antiquity of the groups of rocks, 

 we can scarcely help forming crude ideas of their relative 

 duration, derived from the ascertained thickness of the strata, 

 together with their sedimentary character and organic re- 

 mains. More precise inferences may eventually be made from 

 Astronomical calculations, based on a study of the physical 

 and climatic conditions of each period. This subject has 

 occupied the attention of Dr. Croll, and it may be mentioned 

 that on these grounds it has been inferred that the Glacial 

 period commenced about 200,000 years ago. 



Figures, however, when introduced into Geological Time, 

 must be received with much caution, and the safest position, 

 even with regard to the antiquity of Man, is that recom- 

 mended by Prof Prestwich. That we must greatly extend 

 our present chronology with respect to the first existence of 

 man cannot be questioned. " Nevertheless, just as, though 

 ignorant of the precise height and size of a mountain-range 

 seen in the distance, we need not wait for trigonometrical 

 measurements to feel satisfied in our minds of the magnitude 

 of the distant peaks, so with this geological epoch, we see and 

 know enough of it to feel how distant it is from our time, and 

 yet we are not in a position at present to solve with accuracy 

 the curious and interesting problem of its precise age." ^ 



1 Phil. Trans. 1864, p. 303. On the subject of Geological Time, see J. Croll, 

 Climate and Time, 1S75 ; J. Phillips, Address Geol. Soc. i860, p. 26 ; Sir \V. 

 Thomson, Trans. Glasgow Geol. Soc. vol. iii. ; T. H. Huxley, Address Geol. 

 Soc. 1869; J. C. Ward, G. Mag. 1869, p. 8 ; T. McK. Hughes, Proc. Roy. Inst. 

 1876 ; C. Lloyd Morgan, G. Mag. 1878, p. 145; and T. M. Reade, Chemical 

 Denudation in Relation to Geological Time, 1879. 



