640 W. H. SHERZER RECOGNITION OF TYPES OF SAND GRAINS 



other extreme of the series (see figure 3^ plate 45, and figure 1, plate 46). 

 Sands derived directly from ocean and lake beaches or from river flats 

 are seized by the winds and heaped into dunes^, furnishing us abundant 

 examples of the seolo-aqueous type of sand. These are generally referred 

 to as "dune sands/' but since they are often found not in dunes and 

 dunes occur most abundantly and typically in desert regions, where the 

 source and nature of the granules may be markedly different, the genetic 

 term asolo-aqueous is to be preferred. Owing to their location, they may 

 be returned to the water many times in their history and successively 

 submitted to the action of current, wave, and wind. Typically these sands 

 show a larger admixture of rounded and subangular granules than will 

 be found in the parent beach and river sands (see figure 6, plate 44, and 

 figures 1 and 2, plate 45). The surfaces are more generally smoothed 

 and begin to show the appearance of ground or frosted glass, and some 

 pitting of the surface begins to be detectable. As their name implies, 

 they are intermediate in character between typical aeolian and typical 

 aqueous sands and may graduate imperceptibly into either, which fact 

 will account for the somewhat conflicting statements of observers con- 

 cerning beach, dune, and desert sands and the very general impression 

 that real distinctions can not be made. 



Thus far we have spoken of only those cases in which the wind action 

 is incomplete, its effect not obliterating that of the previously operating 

 agencies. When there is opportunity for strong and long continued wind 

 action, without too much accession of new material, as in the case of 

 some ancient deserts, the granules become more generally and more per- 

 fectly rounded and we have produced the true ^olian type of granule 

 (see figure 6, plate 47). The bodies of the granules themselves become 

 subspherical or ellipsoidal, their surfaces are dulled, frosted, and minutely 

 pitted, seen best in the coarser granules (see figure 2, plate 46). Zittel 

 has described the surfaces of such granules as polished, but this term is 

 inappropriate if he has in mind the granules in a dry state. ^^ The frac- 

 ture surfaces of the quartz granules are smoothed out, the corners and 

 angles gone, but the surfaces are minutely rough. This rounding con- 

 tinues down to grains much smaller than .1 millimeter, the limit set by 

 Daubree for water rounding, but is not general in any desert sand exam- 

 ined, owing probably, as pointed out above, to accessions of new material 

 by residual action and the continuous action of the residual agencies on 

 the granules capable of further change. Owing to the same reason, the 

 assorting of the grains is not as perfect as it would otherwise appear and 



Beitrage zur Geologie der libyschen Wiiste. Palaeontographica, Bd. xxx. 



