h 



THE DESERTS OF AFRICA AND ASIA. 369 



accumulation of sands. It closes the fourth and last stage of its long 

 geological history, without speaking of the different climatic and topo- 

 graphical modifications of quite recent times. This history, as it has been 

 shown, proves that there can be no longer question of a recent rising 

 of the whole Sahara from the bottom of the sea. It is true the Lybian 

 desert is probably somewhat younger than its Saharan sister, for tertiary 

 uncovered deposits (eocene and miocene) have there a larger development 

 than the cretaceous strata ; but, even admitting that the Lybian desert 

 has been upraised after the Miocene period, it cannot be called recent. 



If I have perhaps devoted to the Sahara-Lybian desert more time 

 'than I ought to have done, it is because, among the chief deserts of 

 Africa and Asia, this is by far the best known, so that I have only a few 

 words to say about the Turkestan deserts and the Gobi. 



The two largest deserts of Turkestan are situated between the Syr 

 Daria (Jaxartes) and the Caspian Sea— roughly speaking, between the 45th 

 and 48th degrees of northern latitude, and consequently under the parallel 

 of North Italy and Switzerland. The one — the more eastern — named 

 Kizyl-Kum (signifying in Turkish 'red sand '), is included between the 

 ■Syr Dana and the Amu Daria (Oxus) ; it is limited on the north-west by 

 the Aral Sea, and extends in the southern direction to Bokhara, having 

 from north to south a length of about 400 miles, and from east to wesl; 

 about 300. The other desert, almost of the same development, is situated 

 between the Amu Daria and the Caspian Sea, and extends from the 

 country of Khiva to near that of Merv ; the Turcomans generally call it 

 Kai-a-Kum (black sand). 



I have not visited either of these deserts during my long ramblings 

 m the East, and I am not aware of anything known as for as their 

 geological constitution is concerned. A short notice, however, published 

 about them in the valuable geographical contributions of Petermann.^ 

 <33ntains a few facts which may throw some little light on that subject. 

 The sands which cover certain parts of the Jesert are reported as including 

 shells of mollusks still living in the Aral Set, but where sands are wanting, 

 clay slates (Thonschiefer) form perfectly umovered surfaces. Very good 

 water is found everywhere under the sand it a depth of less than a foot, 

 but it IS reached only at 2 to 4 fathoms in tb clay slates, and is briny and 

 bitter; this difference seems to prove that the sands contain much less 

 salt than the clay slate, probably because the salt spread about among the 

 light quartz particles is more easily dilutee by the atmospheric waters 

 than the salt contained in the compact rock (f clay slate. 



It is most probable that this clay slate behngs to the Palteozoic epoch, 

 and that consequently the two Turkestan deerts have been upraised at a 

 very ancient geological period. As for the saids cemtaining remnants of 

 mollusks still inhabiting the Aral Sea, thej may have been, partly at 

 least, deposited at a time when the Caspiai and Aral formed a single 

 water-sheet. 



In the whole steppe designed by the ollective name of Khiro-is 

 Steppe, of which the Kizil-Kum and the Kira-Kum form only a part, 

 [the climate offers the most violent contrasts. The heat begins in May] 

 when the temperature rises above 122 degres (50° Centigrade) ; and it 

 is precisely under such a temperature that may of the plants peculiar to 

 the sandy and salt soil— as, for instance, the ilhagi camelorum—give to 



• Petermann's MiWmlungen, &c., 1878, v(. xsiv. p. 293, 

 1882. B B 



