552 PRINCIPLES OF STRATIGRAPHY 



ing and pitting of the individual grains, and the kind and character 

 of stratification and cross-bedding. The form of the deposits is of 

 less significance, since that form may be altered or destroyed subse- 

 quently. 



Sorting of Material According to Size and Specific Gravity. A 

 series of experiments was made by J. A. Udden (51) with a cylin- 

 der suspended ninety feet above the ground on a bluff overlooking 

 the Mississippi River, and arranged so as to catch the dust carried 

 by winds of different velocities ranging from 14 miles to 29 miles 

 per hour. From these experiments Udden concluded that "the dif- 

 ferent grades of materials are so far separated from each other 

 in the direction of the wind movement that, even with considerable 

 changes in velocity, the principal area of the deposition of sedi- 

 ments of one grade will not far encroach upon that of the deposi- 

 tion of material much coarser or much finer." Thus, fine gravel 

 or sand will never be carried to the region of the main dust de- 

 posits, and wind-formed sediments will be quite uniform in com- 

 position for any given area. A similar conclusion was reached by 

 von Zittel in reference to the sands of the Libyan desert. (Zittel- 



56-) 



It may thus be stated as the first important character of eolian 



deposits that they are well sorted, according to size and specific 

 gravity of the material, and this means that the deposits of a given 

 locality will consist, to a very large extent, of one mineral type. 

 Thus, an original mixture of grains of various kinds may by pro- 

 longed eolian action be sorted into nearly pure accumulations of the 

 grains of the different minerals. As will be more fully shown be- 

 yond, calcareous particles are thus separated from others and segre- 

 gated into eolian limestones, while clay and other dust particles are 

 removed from the sands which result from the decomposition of 

 igneous rocks, leaving only pure quartz behind. The clay itself 

 accumulates to form deposits of loess. In the Sinai desert, where 

 the quartz is derived from the decomposition of dark-red granite, 

 it represents approximately one-fourth the volume of the original 

 rock. (\Valther-52 : /jj.) The formation of a dune 10 meters 

 high and 20 meters broad results in the accompanying liberation of 

 perhaps 5,000 cubic meters of finest particles of feldspar and horn- 

 blende, which are carried away by the wind. As most of the quartz 

 sand is probably derived from granites, it is apparent that the 

 amount of fine material produced must be enormous. Much of this 

 is carried as dust by the wind. 



Size of grains in eolian deposits. The size of the grains in 

 eolian deposits varies greatly. According to Sokolow, no undoubted 



