700 C. R. KEYES^ MID-CONTINENTAL EOLATION 



ity as an unconsolidated mass to withstand the eroding effects of rain, 

 the presence of unusual amounts of lime, the tendency in deposition to 

 totally disregard existing relief features, are points which are especially 

 noteworthy. Since there is a size below which rounding of grain can 

 not go, the finer materials carried by the wind are apt to be somewhat 

 more angular than the coarser sands. This is well shown by the micro- 

 scope. In contradistinction to formations laid down in water, there is a 

 marked proneness in wind-formed deposits for the flatter grains to lie 

 in all positions, instead of all resting with broad side down. 



The sands of desert regions are characterized by rounded outlines. 

 This is probably the main distinction observable between eolic and 

 aqueous sands. A notable factor in the reduction in size of desert sands 

 is the rapidity of abrasion. On quartz grains, for instance, Mackie®^ 

 has shown that with the most moderate velocities, a water current of 2 

 miles an hour and a wind current of 8 miles an hour, the rounding effect 

 in air is nearly 30 times that for water. Moreover, the rounding by 

 wind affects particles less than one-fifth the size of those abraded in 

 water. The rapidity with which soft minerals are worn is well illus- 

 trated in the vast playa of the Hueco bolson lying beteen the Sierra 

 San Andreas and the Sacramento Mountains, in southern New Mexico. 

 As the shallow waters are completely evaporated each year, a thin layer 

 of gypsum is crystallized out on the bottom of the playa. As further 

 drying takes place the continuous layer of gypsum is cracked and broken 

 into small pieces, which are soon moved along the surface of the ground 

 by the winds and accumulate on the borders of the playa in vast dunes®^ 

 of pure gypsum sands, the particles of which are all perfectly rounded, 

 although the distance traveled has not been more than a few miles. In 

 the same manner dunes composed entirely of rounded salt grains are 

 formed on the borders of many saline lakes, as in the case of the Crater 

 Salt-lake, in western Socorro County, 'New Mexico, and in the Laguna 

 del Perro, on the Estancia plains, in the central part of the Territory. 



The assorting power of the wind so far exceeds that of water that this 

 feature may become an important one in the identification of eolic de- 

 posits. Quartz sands blown by the winds of the desert present remark- 

 able purity. The soften mineral particles are quickly ground to dust 

 and removed through the air. On the lower Volga Eiver, during the 

 excursions of the International Geological Congress in 1897, this circum- 



62 Trans. Edinburgh Geol. Soc, vol. vii, 1897, p. 300. 

 «'Bull. Geol. Soc. America, vol. 19, 1908, p. 84. 



