^OLIAN TYPES 



639 



are the size, hardness, and specific gravity of the grannies, the total dis- 

 tance and the velocity with which they have been moved. Other minor 

 factors that must be reckoned with are cleavability, solubility, tendency 

 to decompose, ability to absorb heat and withstand sndden changes of 

 temperature. The relative ease with which the common minerals become 

 rounded in air, when subjected to approximately identical conditions, is 

 shown in Maddens table quoted on page 634 of this paper, from which it is 

 seen that quartz is still the most resistant of the list, but differing only 

 slightly from orthoclase. 



Sand of the glacial, volcanic, residual, or aqueous type, subjected to but 

 slight wind action, is not to be distinguished from the parent material in 

 isolated samples. Taken in connection with the deposit, however, in 

 which it occurs, a distinction may be made and the prefix ceolo utilized. 

 Furthermore, the evidence of seolian erosion often may be recognized, 

 superposed on the original granule, but without obliterating its main 

 features. To the extent that loess possesses a texture comparable with 

 sand (ordinarily it is much finer) and is derived from glacial deposits by 

 wind action, it represents an geolo-glacial subtype. So far as it may have 

 been deposited by the action of water it would come under the head of 

 an aqueo-glacial deposit. ^^ If the material was largely residual to start 

 with, as advocated by Pumpelly for the great deposits of eastern China, 

 and as supported by the recent researches of the Carnegie Expedition, 

 then such loess should be characterized as seolo-residual,^^ and could 

 probably be distinguished from that of glacial origin. Ordinary volcanic 

 sand must more often than not have its surface layers subjected to wind 

 action, from which there would result sand of the seolo-volcanic variety, 

 recognizable by the composition of the granules, the more or less com- 

 plete rounding of the finer materials, and the nature of the deposit in 

 which found. Much of the sand of the Sahara and other great deserts 

 of the earth is probably geolo-residual in character, resulting from the 

 breaking down by wind erosion and weathering of beds of crystalline and 

 fragmental rocks. ^^ The constant supply of this material, combined with 

 the continuation of the residual agencies, often causes a puzzling mixture 

 of typical seolian sand and sharp, angular fragments which belong at the 



^ A description of the microscopic appearance of a loess from Muscatine, Iowa, is 

 given by Diller in U. S. Geological Survey Bulletin No. 150, p. 65. More general de- 

 scriptions will be found in tlie Sixth Annual Report of the U. S. Geological Survey, 

 1885, p. 278, by Chamberlin and Salisbury. See also McGee's paper in the Eleventh 

 Annual Report, 1891, p. 291. 



25 Pumpelly : The relation of secular rock-disintegration to loess, glacial drift, and 

 rock basins. American Journal of Science, 3d ser., vol. xvii, 1879, p. 133. 



Willis, Blackwelder, and Sargent : Research in China, vol. i, 1907, p. 242. 



28 "Das Gesetz der Wtistenbildung," Walther, 1900, p. 120. 



