WATER AS A MECHANICAL AGENT. 



19a 



The torrential stream in its flood-time cuts channels through, the cone that 

 later quiet depositions fill up. In Fig. 182 a cone is encroached upon (near d) 

 by the river. Alluvial cones, of great size and low angle, occur at the base of 

 the mountains in the Great Basin and in some other parts of the Rocky Moun- 

 tain region, and have been described by Gilbert (1877-1890), Button (1880), 



181. 



182. 



Alluvial cone or fan-talus of upper Indus Basin. 



Triple alluvial cone, ibid. Drew. 



and I. C. Eussell (1885). The gravelly deposits of this kind at the mouth of 

 tributaries in the Connecticut valley and elsewhere were called deltas by 

 E. Hitchcock, and the terraces over the surface either side of the stream, 

 delta-terraces. 



2. Loess. — The terrace-like deposits along portions of the valleys of the 

 Ehine, Danube, and Mississippi consist of loamy earth called loess, which is 

 peculiar in its absence of stratification, and often also in its vertical surfaces 

 of fracture. They have remarkable extent along the Hoang Ho in China. 

 The accompanying sketch, from Richthofen's great work on China (1877), 

 shows its usual landscape features. Erosion reduces portions of its margin 

 to a collection of towers, peaks, and deep and narrow labyrinthine passages ; 

 and human contrivance makes dwelling-places by excavation. The thickness 

 is stated to be in some places 2000 to 2500 feet. The material is a brownish 

 yellow earth, containing land-shells and calcareous concretions. It occurs at 

 several different levels along the river, 100 to 250 feet within 175 miles of 

 the sea ; next, beyond a region of mountains, 1800 to 3500 feet ; after passing 

 another mountain region, 4500 to 5800 feet ; and it is stated to extend to 

 the most western sources of the river over 900 miles from the coast. The 

 river at these levels, as in other cases of loess deposition, was probably lake- 

 like. Long-sustained floods of the rivers in the mountains from melting 

 glaciers are one explanation of the source of the material. Eolian drifting of 

 dust from the salt-steppes of Siberia is Baron von Richthofen's theory, which 

 the absence of a wind-drift structure renders improbable. 



Deposits occur in the Great Basin resembling the Icess in absence of stratification and 

 other characters, which are called adobe by Mr. I. C. Russell, from the name for sun- 

 burnt brick, because this material is used for making the brick. It has usually a 

 yellowish color, and is more or less calcareous. It is described as a result of the wash 



