TRANSACTIONS OF THE SECTIONS. 1:6S 



It appears probable that the alluvial desert plains have been formed in lakes which 

 existed -when the rainftxll was greater than it now is. Around the borders of the de- 

 serts are remarkable slopes of coarse gravel, formed probably of material washed from 

 the surrounding hills. But the great depressions of the country must have been 

 formed under diflerent meteorological conditions, and were probably at one time 

 river-valleys closed by the elevation of ranges of hiUs in the later Tertiai-y period 

 accompanied by a decrease in the rainfall. The desiccation of the coimtry has pro- 

 bably been gradual ; it is possible that in historic times the rainfall was greater than 

 it now is, and that the former population of the country was larger. The change 

 has in all probability been gradual from river-valleys to enclosed lakes and from 

 lakes to deserts. 



It appears probable that a similar change has taken place throughout a large por- 

 tion of Central Asia. A large part of Central and Western Asia, from the Black 

 Sea to Thibet, closely resembles Persia in its physical characters ; and the diying-up 

 of the lower course of the Oxus may have been primarily connected with the 

 diminution of the river due to the decrease in the supply from rain. 



On the Physical Geogmpliy of the Mediterranean, considered in relation to 

 that of the Blade Sea and the Caspian. By William B. Cakpenxer, 

 M.D., LL.D., F.E.S. 



Taking as his datum the equality between the evaporation from the surface of 

 the Caspian Sea, and the amount of fresh water returned to it by rain and rivers 

 (see p. KJo), the author showed the applicability of this datum to prove the 

 cori'ectness of Dr. Halley's doctrine, that the surface in-current of the Strait of 

 Gibraltar is due to the excess of evaporation in the Mediterranean area — a doc- 

 trine which has been recently called in question by Prof. Huxlej^, who has ex- 

 pressed the opinion that, looking to the enormous amount of fresh water poured 

 into this basin by the rivers which discharge themselves into it, " the sun must 

 have enough to do to keep the Mediterranean down." The area of the Black Sea 

 (including the Sea of Azov) and that of the Caspian are nearly equal, each being 

 estimated at about 180,000 square miles. They lie for the most part between the 

 same annual isotherms of 60° and 50°, the extensions of the Caspian to the south 

 of the former and to the north of the latter being nearly equal ; and hence we may 

 conclude that the evaporation from the two seas is nearly the same. Now, as the 

 whole water of the \olga and of the other rivers that empty themselves into the 

 Caspian is only sufficient to make up for its evaporation, it is obvious that the con- 

 tribution of the Danube, the Dnieper, the Dniester, the Don, and other rivers that 

 empty themselves into the Black Sea, towards the supply of the MediteiTanean, is 

 only the excess which remains after compensating for the evaporation of the Black 

 gea — or (assimiing the equality of this with the evaporation of the Caspian) the 

 excess of the volume of the Black-Sea rivers over that of the Caspian rivers, which 

 (as will presently appear) must be a very insignificant contribution to the Medi- 

 terranean in comparison with the area of the latter. 



How small that excess reallj^ is, may be gathered from the experiments on the 

 Dardanelles and Bosphorus cm-rents, of which the particulars have elsewhere been 

 given (p. 41). For not only is the outward surface-current extremely variable 

 in its rate, and liable to occasional reversal, but, when it is at its strongest, its 

 effect is most counteracted by the inward vmdercurrent. The proportional force 

 and volume of the two currents cannot be estimated from these experiments with 

 anv thing like certainty; but Captain Wharton thinks that the undercm-rent 

 sometimes carries in as much as two thirds of the water that the suri'ace-cm-rent 

 carries out. That it ordinarily returns at least half, may be fairly inferred from 

 the constant maintenance of the average salinity of the Black-Sea water at about 

 half that of Mediterranean water ; since it is obvious that this proportion could 

 not be kept up unless as much salt re-enters the basin by the undercurrent as 

 passes out of it by the upper. Hence, as the salinity of the undercurrent is twice 

 that of the upper, its volume may be taken at about one half; so that the actual 

 excess of outflow will be only about one half of the volume of water that forms the 



