TRANSACTIONS OP THE SECTIONS. 165 



tlie Mediterranean with the rainfall over its area, that the annual excess of the 

 former represents a stratum of 4^ feet ; and the largest estimate of the amount 

 brought in by rivers cannot make up a third of this quantity*. "l 



With such an adequate vera causa as this enormous excess of evaporation, there 

 is no occasion to go in search of any other explanation for the Gibraltar in-current. 

 For it is obvious that if the " marine water-shed " between Capes Trafalgar and 

 Spartel were to be raised 1000 feet, so as to cut off the Mediterranean basin from 

 the Atlantic, the excess of evaporation from its surface would produce a pro- 

 gressive reduction of its level (as has happened with the Caspian), until its area 

 came to be so far restricted as to limit its evaporation to the amount returned to it 

 by rain and rivers. But so long as this communication remains open, so long will 

 an in-current through the Strait of Gibraltar maintain the present level and area 

 of the Mediterranean. That this in-current persists through the winter (which is 

 advanced by Prof. Huxley as an objection to the received doctrine) is easily 

 explained. The temperature of the surface, though reduced to 50 degrees or there- 

 iibouts, is still sufficiently high (especially under dry African winds) to maintain a 

 considerable amount of evaporation ; and it is during the season of this reduced 

 evaporation that the river-supply is least ; for all the great rivers which dis- 

 charge themselves into the Mediterranean basin are at their lowest during the 

 winter months, their upper sources being then frozen up, and it is with the 

 melting of the snows that they become filled again. 



On the Physical Geography of the Caspian Sea, in its relations to Oeologyf. 

 By William B. Carpenter, M.D., LL.D., F.R.S. 



The object of this communication was to make known the most important of 

 the fticts contained in the Report of Prof, von Baer on the Physical Geography of 

 the Caspian — these facts having a special interest for Geologists, and affording also 

 a reliable datum in regard to the relation between the amount which is lost by 

 surface-evaporation and that which is returned by rain and rivers. 



The Caspian, which is the largest existing Inland Sea without any outlet, is a 

 " survival " of that gi-eat central sea which, at no remote geological period, 

 covered a large part of Northern Asia ; the gradual upheaval of the land having 

 separated it from the Euxine on the one side, and from the Sea of Aral on the 

 other, as well as from the Arctic Sea, Avitli which this marine province was 

 formerly in communication. How small an elevation has sufficed to cut off this 

 communication on the northern side, is shown by the fact, that the connexion of 

 the Dwina with the Volga by a system of canals has opened a way for vessels to 

 pass between the Caspian and the White Sea. Thus remaining isolated in the 

 midst of land, the Caspian has undergone a series of very remarkable changes, 

 which can be distinctly traced out. 



In the first place, it is evident (as was long since pointed out by Pallas) that 

 the former extent of the Caspian was much greater than its present area. Tlie 

 southern portion of its basin, which lies among mountains whose escarpments 

 extend beneath the water, is by far the ^deepest, a large part of its bottom lying 

 between 2000 and .3000 feet below the present surface of the water. The middle 

 portion has also a considerable depth on the Caucasian side. But the northern 

 portion is nowhere more than 50 feet deep ; and this depth is continually being 

 reduced by the alluvial deposits brought down by the rivers which discharge them- 

 selves into this part of the basin, notably the Volga and the Ural. These rivers 

 run through an immense expanse of steppes, the slope of which towards the 

 Caspian is almost imperceptible ; so that if the level of its waters were to be 

 raised even very slightly, an expanse of land at least equal to its present area 

 would be covered by it. Now, as the present level is about SO feet helorj that of 

 the Black Sea, whilst ample evidence that the steppes were formerly overflowed 

 by salt water is afforded by beds of marine shells, as well as by the persistence of 



* Sir John Herschel, adopting somewhat different data, came to a conclusion essentially 

 the same (' Physical Geography,' p. 27). 

 t Read in Section C. 



