506 



ISLAND LIFE. 



[part II. 



important to inquire whether these processes are either of them 

 so excessively slow as has been supposed, and I devote a chapter 

 to the inquiry. 



Geologists have measured with some accuracy the maximum 

 thickness of all the known sedimentary rocks. The rate of 

 denudation has also been recently measured by a method which, 

 if not precise, at all events gives results of the right order of 

 magnitude and which err on the side of being too slow rather 

 than too fast. If, then, the maximum thickness of the known 

 sedimentary rocks is taken to represent the average thickness 

 of all the sedimentary rocks, and we also know the amount of 

 sediment carried to the sea or lakes, and the area over which 

 that sediment is spread, we have a means of calculating the 

 time required for the building up of all the sedimentary rocks 

 of the geological system. I have here inquired how far the 

 above suppositions are correct, or on which side they probably 

 err ; and the conclusion arrived at is, that the time required is 

 very much less than has hitherto been supposed. 



Another estimate is afforded by the date of the last glacial 

 epoch as coincident with the last period of high excentricity, 

 while the Alpine glaciation of the Miocene period is assum.ed to 

 have been caused by the next earlier phase of very high excen- 

 tricit}^ Taking these as data, the proportionate change of the 

 species of moUusca affords a means of arriving at the whole 

 lapse of time represented by the fossiliferous rocks ; and these 

 two estimates agree in the order of their magnitudes. 



It is then argued that the changes of climate every 10,500 

 years during the numerous periods of high excentricity have 

 acted as a motive power in hastening on both geological and 

 biological change. By raising and lowering the snow-line in all 

 mountain ranges it has caused increased denudation ; while the 

 same changes have caused much migration and disturbance in 

 the organic world, and have thus tended to the more rapid 

 modification of species. The present epoch being a period of 

 very low excentricity, the earth is in a phase of excejptionol 

 stability both physical and organic ; and it is from this period 

 of exceptional stability that our notions of the very slow rate 

 of change have been derived. 



