196 



DYNAMICAL GEOLOGY. 



and deposition of the epliemeral streams and the thousands of little rilla that are 

 occasionally at work over the surface of the dry regions : the annual precipitation is less 

 than 20 inches. The deposits in some places are hundreds of feet in depth. The 

 calcareous portion is attributed to land-shells. It is various in composition, containin<r 

 1 to 14 per cent of alumina, 19 to 67 of silica, and 2 to 5 of water, with 3 to 60 per cent 

 of calcium carbonate. 



183. 



Loess formation on the Hoang Ho, in the province of Shan-Si, China. Richthofen. 



Fine mud-like deposits are formed over the Great Basin in temporary lakes, callea 

 playas, produced by the overflow of rivers, the material of which is related to the preced- 

 ing. The mud contains more or less of the saline ingredients of the evaporating waters. 



(3) Delta-formations. —The larger part of the detritus of a river is carried 

 to the ocean, or lake, into which it empties; and it goes to form more or less 

 extensive flats about the mouth of the stream. Such flats, when large and 

 intersected by a net-work of water-channels, are called deltas; they are river- 

 made, and reach a large size only where the tides are quite small, or are 

 altogether wanting. 



The spread of a river into a delta at its mouth is a consequence of its 

 enfeebled or decrepit state. Deposition is excessive and becomes an obstruc- 

 tion to the flooded river, and consequently, besides keeping open one or two 

 main channels, the waters cut new channels at flood-times, which may partly 

 disappear and become replaced by others in future floods. The surface 

 thereby becomes intersected by many lines of sluggish waters, small and 

 large, which flood-time puts into temporary activity. The deposits have a 

 slight slope seaward, and thus approximate in character to an alluvial cone 

 (Gilbert), although a consequence of the floods of a stream in decrepitude, 

 and not of one in a torrential or vigorous state. Through the flood-deposi- 



