78 



Kum, and at Dshideli MUSHKETOW found Barchan-sand with 

 grains not exceeding 0,i millimetre. 



The sand always, in the Barchans and elsewhere, consists 

 of quartz. A little mica is found in it, also varying quantities 

 of clay (up to 30 40 per ct. on barchans in dry river- 

 valleys), and frequently iron, small quantities of gypsum, cal- 

 cite, etc. (RADDE 1899 p. 16). 



The form of the Barchans has already been described 

 (p. 7). RADDE'S comparison of a barchan- landscape to a frozen 

 stormy sea is a good simile except that waves are not regularly 

 crescent-shaped. Standing on one of the summits and looking 

 towards the north so that the concave sides of the barchans 

 are turned towards one, this imposing waste is most awe-in- 

 spiring. As far as the eye can reach vawe rises behind wave, 

 crest behind crest. The barchans arise irregularly, often se- 

 veral in a group, their flanks blending so that the sharp crest- 

 lines undulate up and down, in and out. Even a gentle breeze 

 raises the fine sand from every crest, and the brownish sand- 

 smoke from the bare dune -summits adds an additional 

 weirdness to the waste landscape. 



As the prevailing winds are northern or north-easterly 

 the sand migrates towards the south and west and crosses 

 the Amu Darya. This will be further dealt with in chap. 11. 



2. Hummock-Desert, Hummock-Sand (in RADDE "Hiigel- 

 sand", in SEMENOW "desert de sable mamelonnee"). Rounded 

 hills, quite low or fairly high, up to about 10 metres, with 

 basin-shaped hollows between them, and with no windward 

 and lee side. They are stationary dunes with a compara- 

 tively rich vegetation. 



3. Desert of the sand-plains ("Sand -steppe", "steppes 

 sablonneuses"), flat or somewhat undulating areas of sedentary 

 sand. 



4. Dune-chain sands ("Ketten-, Reihen-, Wall-, oder Streifen- 

 sand" of RADDE, "deserts de sables en sillons" of SEMENOW). 

 Parallel sand-hills formed by the grey or white sand of pre- 

 sent or past times. Between the chains are valleys, ab. 45 200 

 metres (150700 feet) wide, the soil of which is bare clay 

 (Takyr). The valleys are crossed by lower dunes which 

 run transversely to the main dune-chains. Dune-chain sands 



