RIVERS. 9 



barrens of drifting sand. Hills may be formed in this manner thirty 

 to forty feet in height. In the nearly rainless regions of the interior 

 of our continent, high winds, laden with sand and gravel, are a power- 

 ful agent in sculpturing the rocks into the fantastic forms so often 

 found there* In such regions, also, extensive deposits of wind-borne 

 particles are sometimes found. This is especially true in the interior 

 of Asia and in China, where, according to Richthofen, such deposits 

 are hundreds of feet in thickness and cover thousands of square miles. 

 The geological importance of dust- deposits has only recently been ap- 

 preciated. 



CHAPTER II. 



AQUEOUS AGENCIES. 



The agencies of water are either mechanical or chemical. The 

 mechanical agencies of water may be treated under the threefold aspect 

 of erosion, transportation, and sedimentary deposits. We will consider 

 them under the heads of Rivers, Oceans, and Ice. Under chemical 

 agencies we will consider the phenomena of chemical deposits in 

 Springs and Lakes. 



^ i Rivers Erosion, Transportation, Deposit. 



Mechanical. < Ocean " " 



Aqueous agencies. ■{ ( Ice " " 



Chemical, i ?P rin S s Deposit in. 



Lakes . 



Section 1. — Rivers. 



Under the head of river agencies we include all the effects of circu- 

 lating meteoric water from the time it falls as rain until it reaches the 

 ocean : i. e., all the effects of Rain and Rivers. 



Water, in the form of vapor, fogs, or rain, percolating through the 

 earth, slowly disintegrates the hardest rocks. Much of these percolat- 

 ing waters, after accomplishing the work of soil-making, already treated 

 in the preceding chapter, reappears on the surface in the form of springs, 

 and gives rise to streamlets. A large portion of rain-water, however, 

 never soaks into the earth, but runs off the surface, forming rills, which 

 by erosion produce furrows. The uniting rills form rivulets, which 

 excavate gullies. The rivulets, uniting with one another and with the 

 streamlets issuing from springs, form torrents, which in their course 

 excavate ravines, gorges, and canons. The uniting torrents, finally issu- 



* Gilbert, U. S. Geographical Surveys — Lieutenant Wheeler in charge, vol. hi, Geology, 

 p. 82. 



