T. BELT ON THE STEPPES OF SOUTHERN RUSSIA. 853 



M. tapiroides. In the Sarmatic beds we have evidence of a great 

 change in the conditions of the Vienna and Aralo-Caspian areas. 

 The communication with the Mediterranean sea was interrupted, 

 and an immense isolated basin formed, in which the Sarmatic fauna 

 was developed. This change to an isolated area, and probably a 

 slight freshening of the water of the basin, was followed by a great 

 change in the molluscan fauna ; and we have the unusual geological 

 phenomenon presented of a nearly complete replacement of the 

 species of mollusks, whilst the land mammals remained the same, 

 the two species of Mastodon found in the marine beds continuing 

 through the Sarmatic period. 



The next stage shows a further great freshening of the water of 

 the Aralo-Caspian area (including that of the Danube and the Black 

 Sea), and another corresponding change in the fauna of the Great 

 Lake. The more purely marine genera, such as Mactra, Tapes, 

 Buccinum, &c, disappear, and Dreissena and abnormal forms of 

 Cardium become the dominant groups, accompanied by freshwater 

 genera, such as Paludina, Gi/das, and others. The abundance of the 

 shells of Dreissena or Conge ria has caused its deposits to be named 

 the Congerian strata. It is the Older Steppe Limestone of Murchi- 

 son ; and its characteristic mammals are Mastodon longirostris and 

 Hippotlierium gracile. The same mammals also characterize the next 

 stage, when the western part of the Aralo-Caspian area was entirely 

 freshened, and the Paludina- and Melanojisis-be&s were deposited 

 above the Congerian *. 



From the marine Miocene up to and including the Melanopsis-he&s 

 the strata follow each other conformably ; but above the latter 

 there is a great break in the succession. No representatives of our 

 "Crags" are known in the Vienna basin or in the Aralo-Caspian 

 area, nothing, indeed, that can be classed as Pliocene. The wit- 

 nesses of the great interval of time between the Miocene and the 

 Quaternary are not actual deposits, but the great denudation of the 

 Miocene beds that took place during it. As during this time there 

 was no intrusion of the Mediterranean fauna, it is probable that the 

 great basin drained to the north, either to the east or the west 

 of the Urals, through some channel now concealed beneath the 

 diluvium. 



And this channel appears to have been gradually deepened, so as 

 to lower the surface of the lake, not only to the present level of the 

 sea, but below it ; for the early part of the Quaternary period 

 is marked here, as it is in other parts of the world, by the rivers 

 running at lower levels, and cutting their channels deeper than 

 their present beds. Dr. Peters has informed me that between the 

 Danube and the Theiss, clays, with freshwater shells, are found at a 

 depth of 180 feet below the beds of these rivers ; and Murchison 

 mentions the occurrence at Astrakhan of freshwater shells of existing 

 species 300 feet below the town f. I have followed Mr. Alfred 



* Th. Fuchs, Band lxxiii. der Sitzb. der L Akad. der Wissensch. 1. Abth., 

 Janner Heft, Jahrg. 1876. 

 t Op. cit. p. 573. 



Q.J. G.S. No. 132. 3 k 



