224 ISLA^^D LIFE 



and more powerful oceanic currents, all producing more 

 rapid denudation, At the same time, the internal heat of 

 the earth being greater, it Avould be cooling more rapidly, 

 and thus the forces of contraction — which cause the 

 U23heaving of mountains, the eruj^tion of volcanoes, and the 

 subsidence of extensive areas — would be more powerful 

 and would still further aid the process of denudation. 

 Yet again, the earth's rotation was certainly more rapid 

 in very remote times, and this would cause more impetuous 

 tides and still further add to the denuding 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 jDOwerful 

 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 actino; toofether, 

 their combined effects may have been by no means un- 

 imjDortant. 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 in- 

 tensifying 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 Geologieal Time. — It 

 is not of course supposed that the calculation here given 



