224 



SPRINGS OF WATER. 



nites, and many genera of testaceous animals, that have left their re- 

 mains in chalk and the lower strata, appear to have been extinct be- 

 fore the deposition of the London clay. Nautilites are however, 

 found in it, similar to the species inhabiting the Indian Ocean, and bi- 

 valve and univalve shells are so numerous, that it would be difficult to 

 select any particular species, as peculiarly characteristic of this for- 

 mation. The shells belong mostly to genera inhabiting our present 

 seas ; yet slight variations of form may be perceived, which have 

 induced naturalists to regard them as distinct from living species. 



The springs that rise in the London clay are generally impregna- 

 ted with sulphate of iron and sulphate of lime, and some of the 

 springs contain sulphate of magnesia ; the quality of the water, how- 

 ever, varies much in different situations, and at different depths. To 

 obtain soft water, it is necessary to bore or sink through the London 

 clay to the sand above the chalk, and sometimes into the chalk itself.* 

 The London clay and the under beds have been perforated to the 

 depth of three or four hundred feet in some situations, before good 

 water could be obtained ; when the stratum is pierced which holds 

 the best water, it rises almost immediately, and sometimes overflows 

 the surface. This admits of an easy explanation, by referring to the 

 section of the Vale of Thames. (Plate IV.) The water which en- 

 ters the edges of the porous strata, say at x oc, descends to the low- 

 est part of the trough or basin, and when perforated would rise to 

 near the level of x x, were the strata deposited in a circular basin, 

 the edges of which rose on each side from the Vale of Thames ; 

 but the strata are deposited in a longitudinal basin or trough between 

 the chalk hills of Hertfordshire and Surrey, and the river Thames 

 cuts through the porous edges of the strata below Greenwich j so 

 that the water being there let out, can seldom rise in wells much 

 •above the highwater mark. Were it not for this, we might have nat- 

 ural ye<5 d^eau of considerable height and magnitude in all the squares 

 of London, to cool and refresh the air during the summer months, 

 and supply the inhabitants in the vicinity with salubrious water. In 

 order to preserve the water pure, that is obtained from chalk or the 

 sand over chalk, it is necessary to line the inside of wells, or to put 

 down tubes, to prevent the water from the London clay, intermixing 

 with the pure water from below. 



* At the village of Wildsen, three miles north-west of London, the boring for 

 water was made two hundred and eighty feet into the clay, and seventy-five feet 

 below it into the chalk, when the water immediately rose to within thirty-five feet 

 of the surface. Chalk rocks, and other calcareous rocks in which the strata are 

 divided by fissures that are not filled with clay, always contain water in the fis- 

 sures when the strata dip under the surface of the ground, or when they are cov- 

 ered by argillaceous beds. This is also the case with coal strata ; and the presence 

 of water is necessary to keep the coal in good condition. If the water be entirely 

 drained from a bed of coal a considerable time before it is worked, the, quality of 

 the coal is much deteriorated. This may be occasioned by air penetrating the fis- 

 sures, and promoting the decomposition of pyrites in the coaL 



