228 



ISLAND LIFE. 



[part I. 



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 on so 

 difficult and exceedingly 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 calcalations 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. 



Conduding Bemarks. — In the present chapter I have endea- 

 voured to show that, combining the measured rate of denudation 

 with the estimated thickness and probable extent of the known 

 series of sedimentary rocks, we may arrive at a rude estimate of 

 the time occupied in the formation of those rocks. From 

 another point of departure — that of the probable date of the 

 Miocene period, as determined by the epoch of high excentricity 

 supposed to have aided in the production of the Alpine glaciation 

 during that period, and taking the estimate of geologists as to 

 the proportionate amount of change in the animal world since 

 that epoch — we obtain another estimate of the duration of geolo- 

 gical time, which, though founded on far less secure data, agrees 

 pretty nearly with the former estimate. The time thus arrived 

 at is immensely less than the usual estimates of geologists, and 

 is so far within the limits of the duration of the earth as cal- 

 culated by Sir William Thomson, as to allow for the develop- 

 ment of the lower organisms an amount of time anterior to the 

 Cambrian period several times greater than has elapsed between 

 that period and the present day. I have further shown that, in 

 the continued mutations of climate produced by high excentri- 

 city and opposite phases of precession, even though these did 

 not lead to glacial epochs, we have a motive power well calcu- 

 lated to produce far more rapid organic changes than have 

 hitherto been thought possible ; while in the enormous amount 

 of specific variation (as demonstrated in an earlier chapter), we 



