544 THE WONDERS OF GEOLOGY. Lect. V. 



ductions, did not also exist in mineral waters issuing from 

 strata formed in the ancient seas. 



As the saline springs of the red marls rise up through 

 the Lias (Lign. 121), they undergo certain chemical 

 changes. From the decomposition of the sulphate of iron 

 which takes place, a vast quantity of sulphuric acid must be 

 generated, which reacting on the different bases of magnesia 

 lime, &c. contained in the strata, forms those sulphates so 

 prevalent in the higher or pyritous beds of the Lias ; the 

 oxide of iron being at the same time more or less completely 

 separated. By this means the mineral waters, which are 

 probably mere brine-springs at the greatest depths, acquire 

 additional medicinal qualities as they ascend to the places 

 whence they flow. At the same time it must be borne in 

 mind that fresh water is continually falling from the atmos- 

 phere upon the surface of the Lias clays, and percolating 

 through the uppermost strata.* 



6. Conglomerates of the Trias. — The conglomerates 

 of this formation are chiefly composed of pebbles and 

 detritus, derived from the destruction of igneous and meta- 

 morphic rocks, as slate, quartz-rock, granite, porphyry, 

 &c. ; even the fine siliceous sandstones contain a large pro- 

 portion of debris, It would, therefore, appear that the sea 

 which deposited the saliferous group, was bounded by the 

 rocks of whose ruins it is composed ; in like manner as the 

 existence of beaches of flint-pebbles evinces the destruction 

 of former chalk-cliffs. 



The rock on which Nottingham Castle is built, is a con- 

 glomerate formed of the ruins of the ancient rocks of the 

 neighbouring districts. The rounded pebbles of quartz, 



* The origin of the Cheltenham waters was first pointed out by Sir 

 K. I. Murchison, in his elegant volume " On the Geology of the 

 Neighbourhood of Cheltenham," 1834. See the " Silurian System," 

 for a masterly sketch of the Lias and Trias of Worcestershire, Shrop- 

 shire, &c. 



