AS SOURCES or WATER-SUPPLY FOR TOWNS. 265 



rally and vertically through this formation is due to uni- 

 formity of composition. In some parts of Cheshire and 

 Lancashire we may estimate the thickness of the Bunter 

 sandstone at 2000 feet ; in the central counties, at 500 to 

 1000 feet j and throughout this mass of porous sandstone 

 there seldom occurs a stratum which is impervious to 

 water. We find, it is true, here and there, bands of shaly 

 marl, but they are very thin, and never extend over a large 

 tract of ground. Hence it is that wells, properly situated 

 with respect to the dip of the beds and the area of rain- 

 fall, can drain such large tracts, and when of sufficient 

 depth are not likely to be affected by dry seasons. 



3. Filtering power. — Pure sand and gravel are. the most 

 general agents employed in filtering on a large scale ; and 

 by means of the New Red Sandstone and similar deposits, 

 Nature accomplishes the important function of extracting 

 from water, derived from various sources, all noxious im- 

 purities, and returning it back again, through her springs 

 and fountains, clear and fitted for our use. Such is the 

 character of the water derived from this formation. It is 

 generally very free from matter in mechanical suspension, 

 nor is it often impregnated strongly with salts in solution. 

 Carbonate of lime is often present, but in small quantities. 

 Iron, which sometimes occurs in the water raised from 

 wells, can readily be precipitated by allowing the water 

 free access to the open air. Brine is frequently diffused 

 through the Bed Marl, which forms the upper division of 

 the Trias ; yet I have only known of one instance out of a 

 large number of wells where it has been found in the Bed 

 Sandstone. The case referred to occurred at Ordsal, near 

 Manchester, and is mentioned by Mr. Binney, F.B.S., in a 

 paper recently read before this Society. The purity of 

 the water, and its fitness for manufacturing as well as do- 

 mestic purposes, is most satisfactorily shown, from the fact 

 that it is used in bleaching, dyeing, and other works in 



