SALT 



orerlain by a great thickness of mixed salts, rich in compounds of potash and mag- 

 nesia. (See ABBAUM SALTS ; POTASH.) It is notable that the order in which these 

 various salts are superposed upon each other is precisely the order of their relative 

 solubility, and hence the conclusion that the whole deposits represent the salts left by 

 evaporatio'h of the waters of a great salt-lake. 



The natural transition from the consideration of these strata of rock-salt is to those 

 brine springs which generally accompany them, and which have frequently first 

 called attention to the deposits below. It has been noticed that salt springs issue, in 

 general, from the upper portion of the saliferous strata ; cases, however, occur in 

 which the brines are not accompanied by rock-salt, and in which, therefore, their 

 whole saline contents must be derived from the ordinary constituents of the strata. 

 Thus, in England, besides the strong brines of the New Red Sandstone, we have salt 

 springs issuing from the carboniferous rocks. The purest and most saturated brines 

 are, however, found to be those which can be traced to rock-salt beds, and in the 

 foremost rank of these stand the English springs of the Northwich, Middlewich, and 

 Sandbach districts in Cheshire ; of Droitwich and Stoke in Worcestershire ; and of 

 "Weston and Shirleywich in Staffordshire ; and the continental brines of Wiirtemberg 

 and Prussian Saxony. The following is the composition of these saturated brines : 



Solid contents in 100 parts of brine. 



Compared with these may be some weaker and less pure brines, which rise from 

 other geological formations. The brines in the United States come for the most part 

 from Silurian sandstones, but those in the Alleghany Mountains spring from the 

 coal ; and the weak salt springs of Nauheim and Homburg, which can only be called 

 brines because chloride of sodium is their largest constituent, rise from palaeozoic 

 strata. (See Table at top of next page.) 



These weak salt springs are supposed to have no connection with beds of rock- 

 salt, but to obtain their chloride of sodium, in common with the other salts which they 

 contain, from the strata which they permeate. The singular brines of the Alleghany 

 Mountains must obviously pass through strata containing little if any soluble sulphate, 

 otherwise their chloride of barium would be separated as insoluble sulphate of baryta ; 

 and all indeed may be regarded as coming more iinder the head of ordinary mineral 

 waters, which happen to contain rather a large quantity of chloride of sodium. 



The next source of chloride of sodium which demands notice is found in the inland 

 seas, salt lakes, pools, and marshes, which have their several localities obviously 

 independent of peculiar geological formations. They appear to owe their origin to 

 two causes, being due, first, to the formation of lakes upon, and the passage of rivers 

 through, some of the surface deposits of salts already alluded to ; and, secondly, by the 

 cutting off of a portion of the ocean by the elevation of the land, and the consequent 

 formation of an inland lake. To the former cause are probably due the existence of 

 the Lake Oroomiah in the N.W. of Persia, the numerous brino pools of Southern 

 Russia, and the Great Salt Lake of N. America. Tho lake Oroomiah is S2 miles 



