34 The American Geologist. July, 1892 
these. Some of the delta-like deposits of Tertiary age in the 
Himalaya mountains may be of this origin. 
Very similar to and grading into these deposits are the cone 
deltas or alluvial fans of arid regions. They result from the ina- 
bility of the streams to carry their load any farther. This is 
sometimes brought about by the absorption of the water by the 
soil, or by evaporation, when forms resembling gravel slopes are 
produced; but most commonly the decrease in cai'rying capacity 
is the direct result of the decrease in river slope. For this rea- 
son such deposits exist most commonly at the foot of mountains, 
opposite the ravines, forming a cone shaped delta. These alluvial 
fans often unite with one another by means of talus deposits, or 
by the surface wash, or even by actually meeting in their growth.* 
In the discussion of a paper on secular disintegrationf by profes- 
sor Pumpelly, Mr. Gilbert calls attention to these deposits in the 
arid regions of the west, particularly of the Great Basin, which 
are derived by processes chiefly mechanical and involving little 
rock decay. They accumulate in the valleys and at the foot of 
mountains, in alluvial cones needing only cementation to become an 
extensive conglomerate. 
:\^ (c) By Wind. 
Any one familiar with the conditions in the arid regions knows 
of the great importance of the wind as a transporting agent. 
Scarcely a day passes during which more or less sand and dust is 
not blown about; and very frequently great dust storms occur, 
during which much sand is transported, often to a great distance. 
Prof. Pumpelly statesj that in a simoon the driving dust and sand 
hides the country under a mantle of impenetrable darkness and 
penetrates every fabric. It often destroj's life by suffocation, and 
in places leaves a deposit several feet deep. Such wind blown 
material may find its way to the sea direct, or into streams and 
thence to the sea; but more commoni}' it remains on the land. 
The importance of this means of transportation is not fully 
known, for the subject has never been carefully investigated; but 
when it is so studied, it will undoubtedly be found to be a very im- 
portant source of material for the formation of rocks. 
*For a complete discussion of these deposits I refer to the following 
sources: Drew, Quart. J. Geol. Soc, 1873, xxrx, 441-471. Gilbert, Mon- 
ograph I, U. S. Geol. Survey, 1890. 
tBull. Geol. Soc. Am. 1891, 11, 224. 
JSecular Rock Disintegration. Am. J. Sci., 1879, xvii, 133-144. 
