620 TES (G5 (C/E VA WTESEIEIL IONS 
deepened basins and the land exposed and incidentally corru- 
gated, the conditions for the reversal of the atmospheric change 
are propitious. 
If a computation be made of the amount of carbon dioxide 
that would be required to disintegrate the crystalline rock requi- 
site to supply the clastic material for a great epoch of sandstone 
and shale deposition (allowing duly for old clastics used over), 
a competency to exhaust many atmospheric equivalents of car- 
bonic acid will be shown. This, being correlated with limited 
limestone formation, and consequent scant returns of carbonic 
acid from the ocean, seems competent on its side to notably 
change the constitution of the atmosphere in the direction of. 
poverty of carbonic acid. 
The effect of the limestone-depositing epochs upon the 
atmospheric oxygen and nitrogen has been ignored for simplicity. 
While doubtless important, the process does not seem to have 
any such potential consequences as those which attend the 
decisive and regulative factor carbon dioxide. Its discussion 
will therefore not be undertaken here. 
The action of intercurrent agencies has been purposely 
ignored. Some of these are quite obvious. The organic cycle 
of carbonic acid consumption and oxygen liberation by plants, 
and of carbonic acid production and oxygen consumption by 
animals, and the codrdinate fixation and freeing of nitrogen 
alternately, was an ever present factor, and contributed in its own 
way to a constitutional change of the atmosphere. Although 
the cycle is measurably self-supporting, it is not a solution of 
the problem of perpetual motion, and the total result for a pro- 
tracted period is a permanent alteration of the ratios and of the 
absolute amounts of the constituents of the atmosphere. But the 
discussion of this is reserved. 
The periodicity of epochs of great limestone-formation recip- 
rocating with epochs of great land extension has been assumed 
on the basis of a recent discussion. That there were intervening 
*The Ulterior Basis of Time Divisions and the Classification of Geological His- 
tory. This magazine, previous number, pp. 449-462. 
