CHAP. X MEASUREMENT OF GEOLOGICAL TIME 



235 



fuchsias and tree-ferns on the very borders of huge 

 glaciers, descending to Avithin 700 feet of the sea-level. 

 It is not pretended that these estimates of geological time 

 have any more value than probable guesses ; but it is 

 certainly a curious coincidence that two remarkable 

 periods of high excentricity should have occurred, at sucli 

 periods and at such intervals apart, as very well accord 

 with the comparative remoteness of the two deposits in 

 which undoubted signs of ice-action have been found, and 

 that both these are localised in the vicinity of mountains 

 which are known to have acquired a considerable elevation 

 at about the same period of time. 



In the tenth edition of the Principles of Geology, Sir 

 Charles Lyell, taking the amount of change in the species 

 of mollusca as a guide, estimated the time elapsed since 

 the commencement of the Miocene as one-third that of the 

 whole Tertiary epoch, and the latter at one-fourth that of 

 geological time since the Cambrian period. Professor 

 Dana, on the other hand, estimates the Tertiary as only 

 one-fifteenth of the Mesozoic and PalaBozoic combined. On 

 the estimate above given, founded on the dates of phases 

 of high excentricity, we shall arrive at about four million 

 years for the Tertiary epoch, and sixteen million years for 

 the time elapsed since the Cambrian, according to Lyell, 

 or sixty millions according to Dana. The estimate arrived 

 at from the rate of denudation and deposition (twenty- 

 eight million years) is nearly midway between these, and 

 it is, at all events, satisfactory that the various measures 

 result in figures of the same order of magnitude, which is 

 all one can expect w^hen discussing so difficult and ex- 

 ceedingly speculative a subject. 



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 geo- 

 logical time as defined by the fossiliferous rocks, and that 



