SCENERY, SOIL AND THE ATMOSPHERE 571 



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 dis- 

 appear, 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 tem- 

 peratures 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. Eivers and weathering have done the work. There are 

 practically no closed basins, either dry or wet, no interruptions of drain- 

 age, 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 weather- 



