Mr. J. Croll on Geological Time. 141 



the mean excursion of a molecule between its collisions ; and it 

 is, at all events, not likely that it is much more than ^ in the 

 gases experimented on by Professor Maxwell. Hence the mean 

 distance at ordinary temperatures and pressures is probably of 

 the same order as a ninth-metre, which is the millionth part of 

 a millimetre. It is therefore probable that' there are not fewer 

 than something like a unit-eighteen of molecules in each cubic 

 millimetre of a gas at ordinary temperatures and pressures*. 



XIX. On Geological Time, and the probable Date of the Glacial 

 and the Upper Miocene Period. By James Croll, of the 



Geological Surveij of Scotland. 



[Continued from vol. xxxv. p. 384.] 



IT will not do, however, to measure marine denudation by the 

 rate at which the sea is advancing on the land. There is 

 no relation whatever between the rate at which the sea is advan- 

 cing on the land and the rate at which the sea is denuding the 

 land. For it is evident that as the subaerial agents bring the 

 coast down to the sea-level, all that the sea has got to do is 

 simply to advance, or at most to remove the loose materials 

 which may lie in its path. The amount of denudation which 

 has been effected by the sea during past geological ages, compared 

 with what has been effected by subaerial agency, is evidently but 

 trifling. Denudation is not the proper function of the sea. The 

 great denuding agents are land-ice, frost, rain, running water, 

 chemical agency, &c. The proper work which belongs to the 

 sea is the transporting of the loose materials carried down by the 

 rivers, and the spreading of these out so as to form the stratified 

 beds of future ages. 



We have thus seen that geology, alike with physics, is opposed 

 to the idea of an unlimited age to our globe. And it is perfectly 

 plain that if there be physical reasons, as there certainly are, for 

 limiting the age of the earth to something less than 100 millions 

 of years, geological phenomena, when properly interpreted, do 

 not offer any opposition. 



Perhaps one of the things which has tended to mislead on this 

 point is a false analogy which is supposed to subsist between 



* Hence we may see how entirely remote from a state of emptiness that 

 which usually passes under the name of a vacuum chamber really is. If 

 there be a unit-eighteen of molecules in every cubic millimetre of the air 

 about us, there will remain a unit-XV in every cubic millimetre of the best 

 vacuums of our ordinary air-pumps. The molecules are still closely packed, 

 within about an eighth-metre of one another; i. e. there are about sixty 

 of them in a wave-length of orange light. 



