GLACIAL AND VOLCANIC SAND TYPES 629 



but a slight amount of water erosion is obviously not to be distinguished 

 from that found in the till, so far as the granules themselves are con- 

 cerned. Embedded in the till there occur often lenticular, stratified 

 deposits of such sand, and in the basal layers of the beaches of glacial 

 lakes, which may or may not have been subjected to much aqueous action. 

 Such sand, which shows by the modification of the granules themselves 

 the action of water, may be conveniently referred to as of the aqueo- 

 glacial subtype. 



Volcanic Sand Type 



So far as the general form of the granules is concerned, the sand re- 

 sulting from the action of volcanoes on solid lavas may resemble on one 

 hand that obtained from glacial action. The grains are often very irreg- 

 ular and sharply angular, and give no evidence of erosion about the 

 corners and edges. The assorting is due to gravity acting on the ejected 

 materials while buoyed up by the atmosphere, and may be more or less 

 complete, always more so than in glacial deposits and generally less than 

 that due to water action. The stratification resulting is quite regular 

 and horizontal, and free from any cross-bedding or rippling. In the 

 case of the vitreous materials the sand fragments in their form, luster, 

 and method of breaking may much resemble freshly pulverized glass, 

 seen especially as the sand approaches dust in its texture (see figure 3, 

 plate 43). The coarser particles give evidence of rounding due to mu- 

 tual abrasion while suspended in the air, or, in the case of derivation 

 from molten or plastic lava, from having been thus molded during their 

 aerial flight, and may pass into lapilli (see figure 4, plate 43). Well 

 defined outlines of isolated crystals may often be observed, the corners 

 and edges giving more or less evidence of erosion while in the solid con- 

 dition. The granules derived from volcanic sand generally contain 

 much amorphous material, or show only incipient crystallization, exhibit 

 flowage structure and are more or less vesicular (see figure 3, plate 43), 

 by which they may be readily distinguished from ordinary glacial sand.^ 

 In the case, of a glacier acting on a series of lava beds there may be pro- 

 duced a type of sand very similar, possibly nearly identical, with volcanic 

 sand and to be distinguished from it chiefly by the nature of the deposit 

 in which it occurs. When separated from their parent beds by slight 

 water action the volcanic sand may be expected to show more rounding 



^ A discussion of how volcanic sand may be produced and a description of a sample 

 from the Snag Lake cinder cone, California, is given by Diller in Bulletin No. 150 of 

 the U. S. Geological Survey, 1898, p. 245. See also The eruption of Krakatoa, Report 

 of the Royal Society of London, 1888, p. 38. Russell : Volcanoes of North America, 

 1897, p. 75. 



XLIV— BuLiv, Geol. Soc. Am., Vol. 21, 1909 



