538 ISLAND LIFE 



prove the existence of many and \aried forms of life which 

 require unrecorded ages for their development — ages 

 probably far longer than those which have elapsed from 

 that jDeriod to the jDresent day. The physicists, however, 

 deny that any such indefinitely long periods are available. 

 The sun is ever losing heat far more rapidly than it can 

 be renewed from any known or conceivable source. The 

 earth is a cooling body, and must once have been too hot 

 to support life ; while the friction of the tides is checking 

 the earth's rotation, and this cannot have gone on 

 indefinitely without making our day much longer than it 

 is. A limit is therefore placed to the age of the habitable 

 earth, and it has been thought that the time so allowed is 

 not sufficient for the long processes of geological change 

 and organic development. It is therefore important to 

 inquire whether these processes are either of them so 

 excessively slow as has been supjDosed, and I devote a 

 chapter to the inquiry. 



Geologists have measured Avith 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 Iciiown 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 if 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 excentricity. Taking these as data, the 



