248 POPULAR SCIENCE MONTHLY. 



mightily for a brief time only. While they rest, mountains waste to 

 hills and hills to plains, and the sea spreads over the margins of sink- 

 ing continents. When they put forth their strength, mountains grow 

 and continents rise from the waters. Their intermittent activity 

 exerts a potent influence on the constitution of the atmosphere, and 

 is so important to the hypothesis of glaciation that a more definite 

 account of the evidence of its periodic nature is necessary. 



Sediments laid beneath the sea are waste of continents. By their 

 characters and volume they may indicate their derivation from far- 

 stretching lands or from near mountains. They often occur spread 

 across the bases of ranges which have been planed away by air and 

 waves. By evidences such as these the physical history of any province 

 may be made out; by comparison of provinces the major events in the 

 history of a continent are ascertained; and by comparison of con- 

 tinental histories the sequence of world stages is studied. For any 

 province the limits of knowledge depend only on the completeness of 

 the record in the local rock series ; for each continent the inferences are 

 qualified by difficulties of correlating successive steps from province to 

 province; and world-wide conclusions must necessarily be restricted to 

 the broadest effects. 



In the present condition of geologic investigation we know but 

 incompletely the rhythm of continental and marine oscillation, but cer- 

 tain marked epochs are recognized. Seas were extensive, while lands 

 were low and restricted, during epochs known to geologists as the 

 middle Ordovician, the middle Silurian, the early Carboniferous, the 

 late Jurassic and the upper Cretaceous. At these times the consump- 

 tion of carbon dioxide by rock weathering was comparatively slight, 

 according to hypothesis. On the other hand, lands were wide and 

 seas confined to their basins during the close of the Silurian and begin- 

 ning of Devonian time, during the Permian and early Triassic periods, 

 and during the Pliocene and Pleistocene. These were epochs of 

 unusual consumption of carbon dioxide. 



The climatic effect of depletion of carbon dioxide depends upon 

 the rate at which it is taken from the atmosphere. If it were abstracted 

 slowly a large loss might be compensated by moderate supply, whereas 

 if it were rapidly removed the effect on the atmospheric content might 

 be decided. In this relation the growth of mountains has an important 

 accelerating influence. Although the rate of weathering is conditioned 

 by many factors, elevation is so important that Chamberlin's estimate 

 is probably near the truth. It is that for continental areas the rate of 

 carbonation varies probably more nearly as the square than as the 

 simple ratio of altitudes. 



Modern studies of mountain growth have materially changed the 

 views held within a decade by geologists as to the ages of ranges. 



