0)1 the Genesis of Some Scottish Minerals. 185 



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 wind 

 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 

 quantities of salt which has 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 

 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 S chats of Northern Africa in 

 general, as well as in all other regions where similar geo- 

 graphical conditions obtain. In a region undergoing sub- 

 sidence, layer upon layer of sediments thus impregnated with 

 salt is laid down. Downward percolation of water redissolves 

 it, transfers it to lower levels, and subsequently aids in 

 concentrating it into one bed. 



Eock Salt has hitherto been recorded from only two 

 localities in Scotland. One of these is from Lower Carboni- 



1 Oae estimate places this quantity as high as 350,000 tons per diem. 



