CHAP. X THE EARTH'S AGE 225 



makes any approach to accuracy, but it is believed that it 

 does indicate the order of magnitude of the time required. 

 We have a certain number of data, which are not guessed 

 but the result of actual measurement ; such are, the amount 

 of solid matter carried down by rivers, the width of the 

 belt within which this matter is mainly deposited, and the 

 maximum thickness of the known stratified rocks.^ A 

 considerable but unknown amount of denudation is effected 

 by the waves of the ocean eating away coast lines. This 

 was once thought to be of more importance than sub-aerial 

 denudation, but it is now believed to be comparatively 

 slow in its action.^ Whatever it may be, however, it adds 

 to the rate of formation of new strata, and its omission 

 from the calculation is again on the side of making the 

 lapse of time greater rather than less than the true amount. 

 Even if a considerable modification should be needed in 

 some of the assumptions it has been necessary to make, 

 the result must still show that, so far as the time required 

 for the formation of the known stratified rocks, the 

 hundred million years allowed by physicists is not only 

 ample, but will permit of even more than an equal period 

 anterior to the lowest Cambrian rocks, as demanded by 

 Mr. Darwin — a demand supported and enforced by the 

 arguments, taken from independent standpoints, of Pro- 

 fessor Huxley and Professor Ramsay. 



Organic Modification Dependent on Change of Conditions. 



1 In his reply to Sir W. Thomson, Professor Huxley assumed one foot 

 in a thousand years as a not improbable rate of deposition. The above 

 estimate indicates a far higher rate ; and this follows from the well-ascer- 

 tained fact, that the area of deposition is many times smaller than the area 

 of denudation. 



2 Dr. Croll and Sir Archibald Geikie have shown that marine denudation 

 is very small in amount as compared with sub-aerial, since it acts only locally 

 on the edge of the land, whereas the latter acts over every foot of the 

 mrface. Mr. W. T. Blanford argues that the difference is still greater in 

 tropical than in temperate latitudes, and arrives at the conclusion that — 

 *' If over British India the effects of marine to those of fresh-water denu- 

 dation in removing the rocks of the country be estimated at 1 to 100, I 

 believe that the result of marine action will be greatly overstated " {Geo- 

 logy and Zoology of Abijssinia, p. 158, note). Now, as our estimate of 

 the rate of sub-aerial denudation cannot pretend to any precise accuracy, 

 we are justified in neglecting marine denudation altogether, especially as 

 we have no method of estimating it for the whole earth with any approach 

 to correctness. 



Q 



