CHAP. X.] 



THE EARTH'S AGE. 



217 



power of the ocean. It thus appears that, as we go back into 

 the past, all the forces tending to the continued destruction and 

 renewal of the earth's surface would be in more powerful action, 

 and must therefore tend to reduce the time required for the 

 deposition and upheaval of the various geological formations. 

 It may be true, as many geologists assert, that the changes here 

 indicated are so slow that they would produce comparatively 

 little effect within the time occupied by the known sedimentary 

 rocks, yet, whatever effect they did produce w^ould certainly be 

 in the direction here indicated, and as several causes are acting 

 together, their combined effect may have been by no means un- 

 important. It must also be remembered that such an increase 

 of the primary forces on which all geologic change depends 

 would act with great effect in still further intensifying those 

 alternations of cold and warm periods in each hemisphere, or, 

 more frequently, of excessive and equable seasons, which have 

 been shown to be the result of astronomical, combined with 

 geographical, revolutions; and this would again increase the 

 rapidity of denudation and deposition, and thus still further 

 reduce the time required for the production of the known 

 sedimentary rocks. It is evident therefore that these various 

 considerations all combine to prove that, in supposing that the 

 rate of denudation has been on the average only what it is now, 

 we are almost certainly over-estimating the time required to have 

 produced the whole series of formations from the Cambrian 

 upwards. 



Value of the preceding estimate of Geological Time. — It is not 

 of course supposed that the calculation here given 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, w^hich 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 



1 In his reply to Sir W. Thompson, 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 



