Page:Popular Science Monthly Volume 59.djvu/258

248 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 sinking 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 continental 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 certain 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 consumption 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 beginning 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.