263 MR. E. HULL ON THE NEW RED SANDSTONE^ ETC._, 



It will be observed from the above that out of eight sub- 

 formations four are water-bearing, two are moderately so, 

 and one produces brine, at least generally. 



The excellence of the Triassic and Permian beds above 

 named, as regards their water -bearing powers, depends prin- 

 cipally upon the three following qualities, i. Porosity. 



2. Homogeneity or uniformity of mineral composition. 



3. Filtering-power; to which may be added the occurrence 

 in only very small proportions of lime or other soluble 

 minerals. We shall now briefly discuss each of these 

 qualities. 



I. Porosity.- — This is a quality inherent in all sandstones. 

 In proportion to their greater or less solidity, they allow 

 the passage of water throughout their mass with less or 

 more freedom, and they absorb water with which they are 

 brought in contact to the point of saturation. The sand- 

 stones of the Trias are on the whole of an open and inco- 

 herent character ; and in some districts, where the '' pebble- 

 beds ^' of the Bunter pass into an unconsolidated conglome- 

 rate, as on Cannock Chase, all the rain that falls is imme- 

 diately absorbed, except what is given back by evaporation. 

 Thus the New Red Sandstone is ever receiving, and drink- 

 ing in, supplies from the clouds, which percolate down- 

 wards and accumulate in the lower strata till a water-level 

 is formed, which oscillates according to the rainfall and the 

 extent and number of the springs, which are the natural 

 sluices. When these porous strata rise to the surface, and 

 then plunge beneath the impervious formation of the Red 

 Marl, there can be no doubt they are saturated with water 

 to considerable depths, and to great distances from their 

 outcrop. In this position the water might be reached by 

 artesian wells, were it not for the danger of tapping brine- 

 springs in passing through the formation of the Red Marl. 

 This capacity for forming large reservoirs of water within 

 the mass of the formation is one of its most important 



