EOLIAN DEPOSITS 553 



eolian deposits are known in which the diameter of the grains ex- 

 ceeds 4 or 5 mm. (48) ; more usually it is less than i mm. The 

 sands of the Libyan desert range from 0.5 mm. to 2.0 mm. in 

 diameter with the grains of a given sample of nearly uniform size. 

 Grains of the Sylvania sandstone of Michigan and Ohio, a fossil 

 eolian deposit, vary in average size in different samples from 0.18 

 to 0.3 mm., though an occasional grain 0.5 mrii. or more in diame- 

 ter occurs. Dune sands from Michigan City, Indiana, have an 

 average diameter of grain of nearly 0.3 mm., while various samples 

 from Albuquerque, New Mexico, averaged from this size to 0.13 

 mm. in diameter ( Sherzer and Grabau-45 ; The Sylvania Sand- 

 stone-77). Eolian dust deposits range, of course, downward to the 

 finest particles, which will settle out of the atmosphere, or which 

 may be washed out of it by rains or captured by hygroscopic salt- 

 covered surfaces. 



Rounding of Sand Grains. The process of rounding of sand 

 grains has been discussed at length in Chapter Y, p. 253, where it 

 was also pointed out that wind-blown sands are more perfectly 

 rounded, especially the finer grades, than grains of similar size sub- 

 jected only to water wear. Besides the wearing off of the corners 

 and the general dulling of the surface from prolonged wear, pitting 

 of the grains also results from the violent impact of grain on grain 

 to which they are all subjected. 



In general it may be stated that sand grains of eolian origin 

 are more or less perfectly rounded, and this together with their 

 well-assorted character makes valuable criteria by which such eolian 

 sands may be recognized. In general, the greater the amount of 

 transportation and the more frequent the rehandling of the sand by 

 the wind, the more perfect will be the rounding, as well as the as- 

 sortment. Sands of parts of the Libyan desert, derived from an 

 older sandstone, the Nubian, and carried in some cases a distance 

 of a hundred miles or more, show a pronounced rounding of grain, 

 as well as perfection of assortment, while sands from the desert 

 near Albuquerque, New Mexico, derived from the not very distant 

 crystallines are poorly assorted, and many of the smaller grains are 

 still angular. Sands of shore dunes such as those derived from the 

 glacial sands of Lake Michigan show only moderate rounding, the 

 transportation and wearing having been of comparatively slight ex- 

 tent. Similar conditions occur in dune sands from the Pacific 

 coast, while a like subangular form also persists in sands from West 

 Palm Beach, Florida. Eolian sands along the Nile valley, derived 

 primarily from the disintegration of the crystallines, are not only 

 very poorly assorted, but they also are mostly angular with only 



