EXTENSION OF ARALO-CASPIAN DEPOSITS TO KHIVAH. 
309 
belong to freshwater genera and species. In another memoir, however, by the 
same author l , an opinion is expressed, that though it is now, and has ever been 
but slightly saline, the Caspian is daily becoming more so, both through evaporation 
and the dissolution of masses of salt which occur in or below its bed. To this cause 
M. Eichwald seems to attribute the poverty of its fauna, and the disappearance of 
several species which are supposed to have been in existence at no distant day. 
Even our present amount of acquaintance with the subject would, however, induce 
us to reject this reasoning ; for, in the first place, no valid proof has been given of 
an increase in the saltness of the Caspian ; and secondly, judging from all the fossil 
collections, there is no reason to believe, that the former vast Aralo-Caspian pos- 
sessed a greater variety of species, or that its shells partook more of a marine 
character, than those of the present sea. The truth, indeed, seems to be, that in 
the ancient as in the modern period, the relative proportion of organic contents 
has been perfectly maintained. Now in sheets of fresh water of our own sera the 
number of genera and species is small, but that of individuals great, whilst in the 
ocean their variety is countless, — a result probably due to the great diversity of con- 
ditions produced by strong currents in vast and open seas, and to the removal of 
species from distant countries brought into contact with each other. The persistent 
and almost monotonous zoological character of the Aralo-Caspian limestone is, 
therefore, the best possible proof of the insulated position and uniform, brackish 
nature of the one great inland sea in which it was accumulated. 
This Aralo-Caspian limestone occupies the summits of the extensive isthmus 
between theCaspian and the Aral, M. Eichwald has, as it will be perceived in the 
prefixed table, named two species of shell after this isthmus, the Ust-Urt. Owing 
to the great difficulty of travelling far inland among the wild Turkomans who 
inhabit it, we presume that M. Eichwald has not much examined the interior ; 
but by connecting his zoological determination of the species, which so exclusively 
occupy the shelly limestone forming the eastern cliff's of the Caspian, with the 
independent description of other travellers, there can be no doubt, that the same 
deposit spreads over the whole of the plateau between the two seas, extending to 
Khivah and even far beyond that city 2 . 
> Erganzung seiner friiheren Aiisserangen liber das Verhiil truss des Caspischen zur Schwarzen Meere, 
Erman’s Archiv fur Russland, 1843. 
2 Older tertiary and even secondary and ancient rocks doubtless form the nucleus of the Ust-Urt (see 
note, p. 313). We are now merely speaking of the shelly limestones, marls, &c. of the surface deposits. 
2 s 
