^OLIAN TYPES 



641 



the elimination of the softer minerals is not complete (see fignre 3, plate 

 45, and figure 1, plate 46) . 



Owing to the action of the various residiial agencies on the cleavahle, 

 soluble, or decomposable minerals, combined with the abrasion of the 

 softer ones, they are gradually reduced to dust and blown away, leading 

 to a more perfect assortment and a concentration of quartz, as pointed 

 out by Walther^^ (pages 149-153). Preceding the sand storms in the 

 Libyan desert this observer noted dust clouds Avhich settled about the 

 margins of the desert, and he writes : 



"It is no accident that the bed of vegetationless desert is covered with quartz 

 sand, the bed of the grass-grown steppe with clay dust. It is also no accident 

 that the north African deserts are siTrrounded by clay dust steppes. Desert 

 and steppe belong together not only from the viewpoint of climatology, but also 

 sedimentary formation — the steppe is often the child of the desert" (page 152). 



This removal of the dust and concentration of the quartz was pre- 

 viously described by Sokolow^^ (page 189). In the Astrakhan region 

 he found the sand to consist of 90 per cent quartz, with the grains 

 rounded, irregular, and angular — those entirely rounded being rare, those 

 entirely angular still more rare (page 188). There appears to be a type 

 toward which the seolian sands are slowly working^ but not known to 

 actually attain in any modern desert. Such a sand would be almost per- 

 fectly assorted; would consist practically of quartz, the granules being 

 subspherical, regardless of size ; the surfaces w^ould be dulled and pitted. 

 Such a sand is believed to be shown in figure 6, plate 47. The size of the 

 granules would gradually be reduced until small enough to remain sus- 

 pended by the air currents, when further reduction in size by mechanical 

 erosion would cease. Before this would happen, however, they would be 

 swept away and would enter on another stage of geological history. 



Although the assorting power of the air is superior to that of the 

 water, the deposits resulting therefrom are less regular and even. Ex- 

 periments of Sokolow (loc. cit., page 82), conducted in the laboratory 

 on dry sand, gave an angle of rest ranging from 30 to 40 degrees. If 

 40 degrees was exceeded the sand was very unstable, and became less in 

 ease of rounded granules. His observations of the angles maintained on 

 the lee side of dunes ranged from 29 to 32 degrees. Other observers are 

 quoted: Forchammer having obtained 30 degrees; Andreson, 28 to 31 



28 Walther : Die Denudation in der W^uste und ihre Geologisclie Bedeutung, Abhand- 

 lungen der mathematisch physlschen Classe der Konigl. Sacliiscben Gesellschaft aer 

 Wissenschaften, Band xvi, No. Ill, 1891. See also Udden : Dust and sand storms In 

 the west. Popular Science Monthly, vol. xlix, 1896, p. 655. 



20 "Die Diinen, Blldung, Entwickelung und innerer Ban," 1884. Translated from 

 Russian into German by Arzruni, 1894. 



