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 would 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, 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 . 1 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 
