Page:Popular Science Monthly Volume 76.djvu/575

Rh atmosphere, gravity would seem to be the only agent of change, and its action would be narrow, for disintegrating forces must operate to make gravity effective.

With so much of introduction we are ready to look at the atmosphere in a threefold way—and consider, first, its indirect work; second, its direct work; and, third, its history.

As a means of changing the face of the earth, and of modifying its rocks to a considerable depth, no agency compares with land waters. But we are to remember that waters, if they could exist, could not move without a gaseous medium. Supply our planet with its outfit of oceans, lakes, rivers and underground waters, and, in the absence of a thermal blanket they would be frozen and silent. If they could be conceived as keeping the liquid condition, no transportation of water vapor could take place, no rainfall, and no rivers or glaciers could accomplish their tasks.

Modern geography has introduced the doctrine of the cycle. We mean by this the period in which a continent or any part of it would be reduced from its initial forms of uplift, to baselevel; in other words, the time necessary to wear out a land, and put its waste under the bordering sea. In the course of this wearing out, many land forms—mountains, plateaus, hills, plains, slopes, valleys—would come into being and disappear, in appropriate stages of youth, maturity and age. A great series of evolutionary forms of the land would characterize the passage of a cycle.

The varying amount and condition of land waters give us three types of the geographic cycle and three typical groups of resulting shapes of the surface; these are the normal, the glacial and the arid. The normal cycle is conditioned by medium temperatures and ample precipitation in the form of rains. The glacial cycle exhibits low temperatures and abundant precipitation in the frozen condition. The arid cycle is marked by higher temperatures and low precipitation—so little rain that lakes can not rise to the rims of closed basins due to warping of the crust, for the simple reason that evaporation and soakage take care of the rainfall and no rivers can reach the ocean.

We may illustrate the three kinds of cycle by three well-known parts of our own land. The southern Appalachians show us what happens in normal conditions. There are indeed at least two cycles whose results are clearly shown in this southern region, but for our purpose we simply observe that here are plentiful rainfall and moderate temperatures. Great initial uplifts have given opportunity for land sculpture on a large scale. Rivers and weathering have done the work. There are practically no closed basins, either dry or wet, no interruptions of drainage, and the soil is residual, having been chiefly made by the decay of the rocks in place.

Let us turn to New England. Here the soils are due to the