— 57 — 



the surface of the Clay-desert, therefore no salt, or onlj' a 

 little, crystallises out there. It must be kept in mind that 

 the ground-water in the desert is almost always saline, and 

 that fresh water is only found in the underground streams 

 which have washed out the salts from the water-bearing 

 layers (Baer 1856 p. 47). 



The clay forming the surface of the desert is mostly 

 loess in the southern parts. Pure compact clays (Aralo-Cas- 

 pian formations?) are also said to occur in the Clay-desert 

 (BoRszczow, see p. 25), but I have not seen any. 



The Loess includes a proportion of fine sand, sometimes 

 quite as much sand as clay or even more, and it frequently 

 contains mica. (Obrutshew, cited by Radde 1899 p. 19). It 

 is moreover unstratified, rich in lime, very porous and totally 

 devoid of stones. When water is present, loess is a very 

 fertile soil for vegetation. Below it, at a depth of 7 — 10 

 metres, layers of sand are generally found alternating with 

 thin layers of clay containing gypsum. 



Because of its porous structure loess absorbs water easily, 

 but as the surface-soil is generally compact, that only ab- 

 sorbs water to a slight degree. The result is that the water 

 from precipitation easily runs off, or remains on the top and 

 evaporates. As loess has in addition a high water-capacity 

 (59,5 per ct. of dry weight, Clements), the upper layers re- 

 tain the water absorbed, so that it does not penetrate deeply 

 and is therefore exposed to rapid evaporation^). According 

 to Wysotzki (cited by Ramann p. 402) there is found under 

 the "live" layer, which contains the water of precipitation, 

 a "dead" layer with its water-content unchanged, below this 

 follows the layer containing ground-water (if there is any). 

 Accordingly loess is not favourable to tree-growth as it pre- 

 vents water from sinking down to where the tree-roots can 

 get it (comp, above p. 48). 



It is likewise of importance that loess being a rather 

 fine-grained soil retains the water, so that the roots of plants 

 can only absorb a proportionately small percentage. The per- 

 centage available to the plants Clements calls Chresard, as 



') KOSTYTSCHEFF p. 113, ISMAILSKY p. 24, RaMANN 1905 p. 415. 



