CHANGES IN THE CASriAN. 695 



Changes in the Caspicm. 



The Russian Government has contemplated the estabhshment 

 of a nearly direct water communication between the Caspian Sea 

 and the Sea of Azoff, partly by natural and partly by artificial 

 channels, and there are now navigable canals between the Don 

 and the Volga ; but these works, though not wanting in commer- 

 cial and poUtical interest, do not possess any geographical im- 

 portance. It is, however, very possible to produce appreciable 

 geographical changes in the basin of the Caspian by the diversion 

 of the great rivers which flow from Central Russia. The surface 

 of the Caspian is eighty-three feet below the level of the Sea of 

 Azoff, and its depression has been explained upon the hypothesis 

 that the evaporation exceeds the supply derived, directly and in- 

 directly, from precipitation, though able physicists now maintain 

 that the sinking of this sea is due to a subsidence of its bottom 

 from geological causes. At Tsaritsin, the Don, which empties 

 into the Sea of Azoff, and the Volga, which pours into the Cas- 

 pian, approach each other within ten miles. Near this point, by 

 means of open or subterranean canals, the Don might be turned 

 into the Volga, or the Volga into the Don. If we suppose the 

 whole or a large proportion of the waters of the Don to be thus 



species of mollusca. Several genera and numerous species, wMch are separa- 

 ted by the intervention of only a few miles of land, are effectually prevented 



from mingling by the Cape, and do not pass from one side to the other 



Of the one hundred and ninety-seven marine species, eighty-three do not pass 

 to the south shore, and fifty are not found on the north shore of the Cape." 



Probably the distribution of the species of mollusks ia affected by unknown 

 local conditions, and therefore an open canal across the Cape might not make 

 every species that inhabits the waters on one side common to those of the 

 other ; but there can be no doubt that there would be a considerable migration 

 in both directions. 



The fact stated in the report may suggest an important caution in drawing 

 conclusions upon the relative age of formations from the character of their 

 fossils. Had a geological movement or movements upheaved to different 

 levels the bottoms of waters thus separated by a narrow isthmus, and dislocated 

 the connection between those bottoms, naturalists, in after ages, reasoning 

 from the character of the fossil faunas, might have assigned them to different, 

 and perhaps very widely distant, periods. 



I learn that since the above suggestions were made with regard to the cut- 

 ting cf a canal across Cape Cod, this important work has been actually begun, 

 and is now (June, 1882,) far advanced towards completion. 



