HYPOTHESIS OF CAUSE OF GLACIAL PERIODS 669 
—and this is the true contact area—the increase will appear 
impressive. 
As in other cases, there was here also a self-accelerating 
action. As land was elevated, its underground water level was 
at first carried up measurably with it and lay near the surface. 
Trench-cutting accompanied it, to be sure, but at a slower rate. 
As the declivity increased the cutting and transportive power of 
the drainage increased, and as the dissection of the land pro- 
ceeded, the water level was lowered and the effective zone of 
atmospheric contact augmented. The very process of degrada- 
ion, up to a certain stage, increased the facilities for the chem- 
ical action of the atmosphere. In general it may be observed 
that degradation reacts upon itself favorably for atime. The 
cutting of certain of the western plains into ‘‘bad lands” and 
the gullying of the cultivated fields in certain parts of the south- 
ern states are striking examples of current self-accelerating 
processes of the more mechanical sort. 
Concurrent with this increase of the atmospheric contact- 
area on land there was a reduction of sea surface, the habitat of 
lime-secreting life, and, xofa bene, an almost complete oblitera- 
tion of the epicontinental seas and sea-shelves which were the 
parts of the sea bottom that were by far the most prolific in 
carbonic-acid-freeing marine life. Shallow-water marine life 
must have been very generally driven down on the abysmal 
slopes and on to such limited deeper shelves as may have been 
brought within reach by the lowering of the seas. The conse- 
quent lessening in the rate of freeing of carbon dioxide is 
assumed to have been great, and this codperated with the accel- 
erated consumption on the land to hasten the depletion of the 
atmosphere. 
Besides this, when any appreciable reduction of temperature 
followed these codperating agencies, it tended of itself to check 
the lime-secreting life of the ocean, and at the same time to give 
the oceanic waters greater absorptive power and less dissociative 
activity, thus calling into operation a group of secondary agen- 
cies which intensified the effects of the primary agencies. 
