The Origin of Bock- Salt. 37 



"In the case of Eock-Salt, the primary sources of the 

 mineral may be fourfold. The salt may be simply washed 

 out of some rocks in which it happens to occur;. it may arise 

 through the direct combination of sodium salts with others 

 which yield hydrochloric acid ; or it may be due to the 

 evaporation of shallow pools of sea-water in areas tempo- 

 rarily shut off from the open sea. One contributory cause, 

 which has been generally overlooked, is connected with the 

 important fact that the aqueous vapour present in the 

 atmosphere undergoes condensation most readily upon solid 

 particles ; and that, of all the solid particles present in the 

 air, those of chloride of sodium are in this respect amongst 

 the most potent. The water of the sea is lashed by the 

 winds into foam and spray as it is driven against the rocks ; 

 much of the spray is transported inland by the same agent ; 

 its saline constituents are widely diffused, in the form of 

 extremely minute particles of salt-dust, throughout all the 

 lower strata of the atmosphere ; and, finally, it is largely 

 upon these saline nuclei that condensation eventually takes 

 place. That chloride of sodium is ever present in meteoric 

 waters has been abundantly proved by analysis — as much as 

 thirty-two pounds per annum per acre having been recorded 

 by Angus Smith from rain-water alone. Part, at least, of 

 this is usually returned to the sea by rivers. But in the 

 case of such river- waters as are largely or entirely dissipated 

 by evaporation, the saline constituents are left. The shallow 

 mouths of the rivers draining into the Caspian afford large 

 quantifies of salt which have thus originated ; while at Kara- 

 Boghaz, on the eastern side of that inland lake, where the 

 evaporation is exceptionally high, large quantities of salt are 

 annually deposited. The same occurs in nearly all the lakes 

 of the Aralo-Caspian area, and, indeed, in all cases in which 

 river-water is largely dissipated by evaporation. This is the 

 reason why chloride of sodium occurs in such abundance in 

 Lower Egypt, and in the Schatts of Northern Africa in 

 general, as well as in all other regions where similar 

 geographical conditions obtain. . . . Layer upon layer of 

 sediments thus impregnated with salt is laid down. Down- 

 ward percolation of water dissolves it, transfers it to lower 



