CLIMATE AND TERRESTRIAL DEPOSITS 369 



equilibrium attained. The volume varies throughout the year and 

 it is observed that in accordance with this principle by far the greater 

 proportion of river detritus is moved in the seasons of flood. The 

 same principle may be extended to apply to climatic cycles longer 

 than that due to the revolution of the earth about the sun. (2) If 

 through any climatic or tectonic change the forces of erosion are 

 quickened and either a coarser or larger load of waste is poured into 

 a stream of postulated constant volume, the stream must deposit the 

 coarser of this material in place until the grade is steepened and the 

 velocity consequently quickened sufficiently for transportation of a 

 larger proportion. If the waste, on the contrary, becomes finer in 

 grain or deficient in quantity, the stream responds by beginning to 

 erode in its upper portions, tending to reach a new state of equilibrium 

 by partially increasing its load and partly flattening its grade. 



Relations of Stable Climates to Transportation 



deductive conclusions with confirmatory illustrations 



From the foregoing principles the relations, stated deductively, 

 may be drawn between transportation and climate. The conclusions 

 may then be tested by the examination of various regions which supply 

 the requisite conditions. In sub-arid climates the streams in their 

 upper portions have great temporary volumes and carrying power 

 but the volume does not normally increase on the way. The result 

 is that on escaping from the mountains the rivers in such climates 

 are peculiarly liable to lay down a portion of their load, building 

 piedmont slopes. The methods of erosion in an arid climate and 

 the sudden tumultuous action of the streams result moreover in a 

 large proportion of the deposit, consisting of a coarse and rather 

 undecomposed waste, requiring relatively steep grades for its removal. 

 The silt is swept farther out upon the flatter plains, while the clay 

 and loess-like material may be carried to the front of the delta, pro- 

 vided the river waters are not absorbed on the way. The rivers of 

 Argentina flowing eastward from the Andes furnish excellent exam- 

 ples, and to a lesser degree the rivers crossing the Great Plains of the 

 United States. In both cases a condition of equilibrium is approached 

 though never quite attained, between volume and waste upon a slope 



