406 
ACKROYI) : ON THE CIRCULATION OF SALT. 
let and a rate of evaporation equal to the inflow of water, it would, 
in 50,000 years, contain two and three-quarter millions of tons of 
salt, or if we halve the rate of accumulation it would still be 1,375,000 
tons of salt, collected from a gathering ground of 2,'2'23 acres into a 
reservoir of 93 acres surface, and of a depth of Go feet. The reservoir 
at this lesser estimate would contain 32 per cent, of salt, as compared 
with the Dead Sea with its 24 per cent.,"^ and the accumulation, 
derived nearly entirely from rain-borne sea-salt, and some of it already 
precipitated, would have been amassed during a period which is com- 
paratively trifling, as geological time is reckoned, being probably less 
than a seventh of the Pleistocene Age. 
We have here then a process at work which must be taken into 
account in framing theories of salt-lake and salt-deposit f (urinations ; 
hitherto it has been entirely overlooked, and only contributions of 
earth-salt derived by solvent denudation have been drawn upon for 
the purpose, with, in many cases, the added supposition that the first 
step in their formation was the disconnection of an arm of the sea, 
a supposition which is not always required. Thus, with regard to 
the salt deposits of Northern Africa, Prof. Zittelt has shown on what 
appears to be overwhelming evidence, that the popular idea of the 
Sahara having been the basin of a sea in Pleistocene times is without 
foundation, and as the rainfall was heavier and the climate damper 
in those days than now, rain-borne sea-salt must have played an 
important part in the formation of the salt hills of the Sahara. 
The usual theory of the cause of the saltness of tlie Caspian Sea 
appears also somewhat anomalous. The Mediterranean, Black Sea, 
Sea of Azoi, and Caspian Sea, decrease in saltness in this order, on 
which Ramsay has remarked : J " It will be seen that the Black Sea 
is fresher than the Mediterranean. . . , The Caspian is still fresher, 
and its fauna and fossils in recent deposits in the neighbourhood 
prove it to have once had connection with the Black Sea, from which 
* This figure, often quoted, refers to the "saline residue" in which the 
quantity of common salt, as will presently be sliown, is governed by the 
amount of magnesium salts present. 
t Nature, Vol. XXIX., pp. 121 and 122. 
: Nature, Vol. VIL, p. 313. 
