GEOLOGICAL PAPERS. 113 



mon salt. Tbe waters of the Atlantic show the following varieties 

 and proportion of salts : 



Chloride of sodium (common salt) 77.758 



Chloride of magnesium 10.878 



Sulfate of magnesia 4.737 



Sulfate of lime ( gypsum ) 3.600 



Sulfate of potassium 2.465 



Carbonate of lime 0.345 



Bromide of magnesium 0.217 



loo 



When such a body of water is cut off and evaporated, the gypsum 

 is deposited after 37 per cent, of water is removed, and common salt 

 only after the removal of 93 per cent. The normal order of these 

 formations would be a deposit of gypsum, and then a much heavier 

 deposit of salt. But since 93 per cent, of the water must be evapo- 

 rated before the salt would be thrown down, the evaporation might go 

 far enough for the deposition of gypsum, but not far enough for salt ; 

 or the salt might be deposited and subsequently removed by solution. 

 The first condition apparently took place in the Kansas gypsum area, 

 and both conditions probably occurred in Michigan. Gypsum de- 

 posits are more wide-spread in nature than salt, but they usually oc- 

 cur in thinner beds. 



In most areas, the amount of gypsum found is far greater than the 

 amount that would be found in a body of ocean water sufficient to 

 cover the gypsum area at reasonable depths. The present conditions 

 in the Mediterranean sea seem to aid in explaining the formation of 

 such deposits, and it has been cited for this purpose in the discussion 

 of the Kansas and Iowa deposits, and also in the older reports of the 

 Michigan gypsum. 



The most complete study of the composition and currents of the 

 Mediterranean sea has been made by Captain Nares and Doctor Car- 

 penter, in charge of H. M. S. "Shearwater" in 1871.^ They found the 

 basin of this sea to be 6000 feet in depth, separated from the ocean 

 at the Straits of Gibraltar by a ridge 1200 feet high. The water of 

 the Atlantic outside this ridge had a specific gravity of 1.026; at the 

 western end of the basin the gravity was 1.027, and at the eastern end, 

 1.03. The proportion of salts in the ocean was 3.6 per cent., and in 

 the Mediterranean it was 3.9. Passing over the dividing ridge were 

 two currents, one over the other. The upper was inflowing and the 

 lower outflowing. The water of the basin is not concentrated enough 

 to deposit salt and gypsum, but it is gaining in quantity of salt held 

 in solution. 



1. Published in Proc. Royal Soc, XX, pp. 97, 414, 1872 ; quoted also in Encycl. Britannica, 

 vol. XV, p. 821. 



