538 THE WONDERS OF GEOLOGY. Lect. V. 



sedimentary rocks of an age intermediate between the carboni- 

 ferous and cretaceous formations; with the solitary exception 

 of the Virginian coal-field previously described (p. 520). 



3. Rock-salt and Brine Springs of the Trias. — 

 In England, the Trias is the chief repository of salt or 

 chloride of sodium : and brine- springs, which are sub- 

 terranean streams of water impregnated with salt from 

 percolating through saliferous strata, are abundant in the 

 great plain of the red marls and sandstones of Cheshire. 

 The salt, however, is not uniform in extent, but occupies 

 limited areas. The saliferous strata of Northwich present 

 the following series : — 



Feet. 



1. Uppermost calcareous marl ... 15 



2. Eed and blue clays 120 



3. Bed of rock-salt 75 



4. Clay, with veins of rock-salt . . 31 



5. Second bed of rock-salt . . . .110 



Droitwich, in Worcestershire, which is situated nearly 

 in the centre of the county, has been celebrated for the 

 production of salt from its brine-springs, from the time of 

 the Romans; and ever since, this inexhaustible fountain 

 of saline water has continued flowing up, and yielding salt 

 in undiminished quantities.* It is probable that the manu- 

 facture is coeval with the town itself; but it was not till 

 the year 1725, that the strong brine for which it is now 

 famous, w r as discovered ; from one spring alone, the 

 enormous quantity of a thousand tons of salt are obtained 

 per week. 



At a distance of from thirty or forty feet below the surface, 

 there is a bed of hard gypsum, about 150 feet thick : through 



* The Romans imposed a tax on the Britons who worked the 

 Droitwich salt mines, and made salt a part of the pay of their soldiers' 

 solarium, or salary. Hence the custom of asking for salt at the Eton 

 Montem. — Geology of England and Wales, p. 282. 



