EOLIAN DEPOSITS 549 



shales of North America, notably the Chattanooga shale of Ten- 

 nessee, seem to have been produced in this manner. 



B. ANEMOCLASTIC OR EOLIAN DEPOSITS. 

 ANEMOLITHS. 



Deposits which owe their location and form to transportation 

 and deposition by wind are commonly known as eolian deposits, 

 and when consolidated become eolian rocks or anemoliths. At first 

 regarded as of relatively little importance, they are being recognized 

 in constantly increasing number among the clastic rocks of the 

 geological column, and their thickness, great horizontal extent and 

 often marked structural characters, as well as the evidence they 

 furnish regarding the former physical geography of the region in 

 which they are found, render them of special significance to the 

 student of earth history. 



Source of Material of Eolian Deposits. The ultimate origin 

 of the material entering into eolian deposits is extremely diverse. 

 Only a small part is directly due to the corrasive action of the wind, 

 freighted with sand grains or other material, which act as the tool 

 in the erosive work (see ante, Chapter II). A large part is de- 

 rived from the products of atmospheric decay or is directly pro- 

 duced by insolation, and so is primarily of atmoclastic origin. But 

 the mechanically produced sand and rock flour, whether due to 

 stream or wave work, or derived through the grinding of the rock 

 masses into sand and powder by glaciers, form no mean source of 

 wind-transported material. Not unimportant in this connection is 

 the dust produced by organisms, such as herds of animals pounding 

 a rock mass to powder beneath their hoofs, and, above all, the activi- 

 ties of man in breaking up rocks by mechanical means, and grind- 

 ing to powder the surface of the country under the wheels of his 

 vehicles. The annual amount of dust produced by heavy or rapid 

 traffic over an ordinary road is probably far in excess of the me- 

 chanically formed detritus produced by erosive action of an ordi- 

 nary stream in a bed of similar width. The dust produced in quar- 

 rying and mining operations forms no mean addition to the material 

 furnished for eolian transport, while coal dust, the product of in- 

 complete combustion, sometimes constitutes an important mineral 

 impurity of the air (see ante, Chapter 11). Where rocks are ground, 

 as in cement mills or in stone crushers, the air for considerable dis- 

 tances all around is murky with suspended dust. This settles on 

 and around the vegetation of the neighborhood, but may also be 



