THE ULTERIOR BASIS OF TIME DIVISIONS 459 
reciprocating with crustal movements and quiescence seem to 
me to be well indicated as the master features of geologic prog- 
ress from the beginning of the Paleozoic era to the present time. 
To these features I look for the primary terms of a natural and 
permanent system of classification and nomenclature. 
III. The third agency which affords some promise of becom- 
ing a means for strict correlation of transoceanic events and for 
the division of these events into their natural epochs is an 
assumed fluctuation in the constitution of the atmosphere. Too 
little has yet been learned by direct induction respecting the 
nature of the successive atmospheres of the geologic periods to 
render this a firm ground for conclusions, but I venture to invite 
attention to the doctrine enunciated some time ago’ that the 
exposure of the crystalline areas to the action of the air neces- 
sarily led to changes in the constitution of the atmosphere, 
especially in the critical element of carbon dioxide. The prin- 
ciple was urged that the greater the exposure of the decompos- 
able crystalline rocks in area and in elevation, by leading to 
wider contact and deeper penetration of the atmosphere and 
atmospheric waters, the more rapid must have been the decom- 
position of the crystalline rocks and the consequent consump- 
tion of carbonic acid in the carbonation of the alkalis and 
alkaline earths, which is the most important part of the decom- 
posing process. This greater exposure obviously followed the 
crustal readjustments, for at these times the land was largest 
and highest. It then not only exposed the greatest surface to 
atmospheric contact, but the atmospheric waters penetrated 
deepest because of the hydrostatic pressure arising from great 
differences of water level. At times of approximate degradation 
to baselevel and of sea-border encroachment the area of action 
was reduced and the power of penetration of the atmospheric 
waters became slight because of the low elevation and conse- 
quent slight differential pressure. In a word, the consumption 
of the carbonic acid proceeded rapidly at times of broad and 
™A Group of Hypotheses Bearing on Climatic Changes. Jour. GEOL., Vol. V, 
No. 7, October-November 1897. 
