506 
ISLAND LIFE. 
[part ir. 
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 assumed to 
have been caused by the next earlier phase of very high excen- 
tricity. Taking these as data, the proportionate change of the 
species of mollusca 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 exceptional 
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. 
