GEOLOGIC TIME. 305 



years would be required for the depositiou of tlie known sedimentary 

 rocks. On the theory that the present existing sedimentary rocks 

 have on the average passed at least twice through the cycle of destruc- 

 tion and reformation the jieriod is multiplied by three, which results in 

 72,000,000 years for the time of duration since the beginning of the 

 deposition of the sedimentary rocks. He says further, " It is impossible 

 to tell from geological data the actual age of the stratified rocks ; but 

 this is not required. What we require is, as already stated, not their 

 actual age but an inferior limit to that age." 



Wallace. — The chapter on " The earth's age" contained in Sir A. R. 

 Wallace's Island Life, is an admirable summary of his own views and 

 those of various geologists, naturalists, and physicists who have written 

 on the subject. From the consideration of data bearing on the denuda- 

 tion and deposition of strata as a measure of time he thinks that 

 28,000,000 years will be sufQcient for the deposition of the known sedi- 

 mentary rocks. Of the value of this estimate he says, " It is not of 

 course supposed that the calculation here given marks any api^roachto 

 accuracy, but it is believed that it does indicate the order of magni- 

 tude 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 thick- 

 ness of the known stratified rocks." t 



By adopting CroU's theory of glacial epochs occurring at certain 

 periods of great eccentricity, several datum points are secured by Wal- 

 lace that are correlated with certain geologic phenomena of the Ter- 

 tiary and Pleistocene periods and the probable date of the Miocene 

 period. He then takes the ratio of Lyell for the duration of the geo- 

 logic epochs and concludes that 16,000,000 years have passed since Cam- 

 brian time. On the basis of Dana's theory that the Tertiary is only 

 one-fifteenth of the Mesozoic and Paleozoic combined, the result is 

 60,000,000 years for the same interval. Of these figures he says: 



"The estimate arrived at for the rate of denudation and deposition 

 (28,000,000 years) is nearly midway between these, and it is, at all 

 events, satistactory that the various measures result in 'figures of the 

 same order of magnitude, which is all one can expect on so difficult 

 and exceedingly speculative a subject. The only value of such esti- 

 mates is to define our notions of geological time, and to show that the 

 enormous periods of hundreds of millions of years, which have some- 

 times 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 calculation 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." | 



* Stellar Evolution, and its lielations to Geological Time, 1889, p. 39. 

 t Island Life, 2d ed., 1892, pp. 222, 223. 

 X Loc. cit., pp. 235, 236. 

 SM 93 20 



