ON THE ALLUVIAL DEPOSITS. 375 



merly shown how the decomposition of sandstones 

 produces deep-seated sand beds ; and it is equally plain 

 that superficial ones must generate sands subject to 

 transportation, as the quadersandstein does ; while such 

 is the probable origin of many well-known sands in 

 the north of Europe. But, while the most noted of 

 these are the sandy deserts of Africa and Asia, I have 

 already shown that they have been derived from the 

 red marl : while it is easy to understand how the gra- 

 dual disintegration of the surface must continue to pro- 

 duce that, of which the effects have been described in 

 such terrific colouring. If the presence of salt has 

 given rise to the unfounded opinion that such deserts 

 had been lately covered by the ocean, or were its de- 

 serted bed, I have already answered this ; as I have 

 shown that the shells of these deserts are not marine. 

 The elevation of the great Steppe of Naryn in Tar- 

 tary, is itself sufficient evidence that the present sea was 

 not the parent of these salt sands. It is nevertheless 

 possible that a sand of this nature might belong to the 

 elevated submarine alluvia : as it might thus be called, 

 and perhaps may have been, a " tertiary formation." I 

 do not believe that any geologist will hereafter think 

 thus of the deserts in question^ at least ; while, in any 

 doubtful similar case, attention will be required in dis- 

 criminating the clays and sands arising from such local 

 disintegration. It is from their apparently eternal re- 

 production that the spreading of these sands is ex- 

 plained ; that which the winds remove being replaced 

 by the destruction of fresh rock. Hence the well- 

 known extension of those which are daily burying still 

 deeper the architectural remains of Egypt ; while, if I 

 dare not indulge in this interesting subject, we may 

 thus explain much that appears obscure in antient hia- 

 tory. Palmyra was not always the arid desert that it 



