HYPOTHESIS OF CAUSE OF GLACIAL PERIODS 595 
there is a potential hydrostatic pressure measured by the differ- 
ence of altitude of the surface of the ground water and the 
surface of the sea which tends to cause the waters to flow 
through the rock and thus find a tortuous way to the equilibrium 
of the sea level. A deep underground circulation is therefore a 
function of high altitude, and, correlated with that circulation, 
is chemical activity proportionate to the enrichment of the sur- 
face waters with chemically active agencies, in the present 
instance, carbon dioxide in particular. This deeper circulation, 
correlated with hydrostatic pressure, is enhanced by the greater 
degree of fissuring of the rock which attends elevated tracts, for 
in the process of elevation, writhing and cracking are notable 
incidents, and, in addition to this, the gravitative tensions which 
necessarily attend an elevated position lend their aid in the pro- 
duction and opening of crevices. Precisely the opposite con- 
ditions prevail at low levels. The protruding superficial portion 
of the land is the most fissured part and when it has been cut 
away by erosion the basal remnant is normally less open to the 
penetration of water. It would appear from a consideration 
of these several associated influences that disintegration and 
decomposition are facilitated by elevation in a very important 
degree. 
If, therefore, there were times in the history of the earth 
when there were general readjustments of its bodily form to 
accumulated internal stresses, resulting in extensive elevations, 
embracing not only the formation of mountains and plateaus, 
but the general warping outwards of the continental platforms, 
and the bowing downwards of the ocean basins, attended by the 
withdrawal of the sea, so that the land area was extended and, 
at the same time its average elevation increased, and a portion 
of it rent and crushed, it is believed that the carbonation of the 
rocks must have been accelerated by some notable multiplier 
and that the rate of consumption of the carbon dioxide of the 
atmosphere must have been correspondingly promoted; and 
this is made an important postulate of the hypothesis. 
At least two periods of such very general and notable 
