742 FRANK A. WILDER 



Many writers on gypsum and salt have called attention to the 

 fact that the Mediterranean Sea furnishes conditions which, if 

 but slightly modified, would result in deposits of these substances.^ 

 Although it receives the waters of many rivers, some of them of 

 considerable size, evaporation takes place faster than inflow, and 

 if no water entered through the Strait of Gibraltar, or if the 

 supply entering were considerably reduced, much of the mineral 

 matter held in solution would be deposited. A steady current 

 pours in from the ocean, however, and the density necessary for 

 precipitation is not reached. The bottom of the sea rises sharply 

 near the Strait of Gibraltar, cutting off communication between 

 the lower part of the sea and the ocean, but permitting a free 

 interchange of water in the upper level. The depth at the strait 

 is less than 200 fathoms, while the average depth of the Medi- 

 terranean is 1,000 fathoms. The accompanying diagram roughly 

 illustrates existing conditions: 



Fig. 3. 



The amount of salt in the water of the Atlantic is 3.6 per cent., 

 while in the Mediterranean it is 3.9 per cent. The specific grav- 

 ity of the water of the Atlantic off the Strait of Gibraltar is 1.026, 

 while at the west end of the Mediterranean, near the surface, it 

 is 1.028, increasing in the east end to 1.03. At a depth of 300 

 fathoms the density is considerably greater than at the surface, 

 A current of water flows in constantly at the surface of the strait 

 (Fig. 3, a). This water is concentrated by evaporation and 

 sinks. The bottom below the line c d has been previously filled 

 by this dense water, and the water above this line is being con- 

 stantly condensed and sinking causes a portion at the bottom 

 water to flow out at <: as a lower current into the ocean. The 

 outlet at the strait is so free that the condensation does not reach 

 the point which results in the deposition of lime, gypsum, or salt. 



' Geological Survey of Michigatt, Vol, V (1881-93), P^^t II, pp. I-13; University 

 Geological Survey of Kansas, Yol. 5, "Gypsum," Introduction. 



