20 SMALL WATER SUPPLIES. 



They are, however, much more capable of percola- 

 tion. On the whole they yield good water, but un- 

 fortunately are somewhat liable to pollution from 

 the surface by their great capability of infiltration. 

 The oolitic rocks, also called Jurassic rocks, comprise 

 principally Portland stone, limestone, and calcareous 

 grit, and from a stratigraphical l point of view con- 

 sist of four great masses of calcareous or moderately 

 permeable strata separated by thick deposits of clay. 

 Thus we have alternative pervious and impervious 

 strata which causes many springs, and a copious 

 supply in wells sunk in the impervious, though if 

 the clay has to be pierced great depths may be en- 

 countered before water is reached. Portland stone 

 will only yield water in fissures, but the porous 

 strata below, which in turn rests on a bed of clay, 

 provides much water. 



Then there is the Trias formation. It does not 

 possess many fissures and yields a good supply of 

 hard water, except in cases where it overlies what 

 is known as Bunter' 2 Sandstone, when it will con- 

 tain gypsum and rock salt in solution and in sufficient 

 quantities as to render it unfit for use. The Trias 

 formation consists of red, blue, and white clay, rest- 

 ing on dense red clay and sandstone. It is classed 

 together with the Permian formation and may rest on 

 granite or limestone, and usually lies in horizontal 

 beds. Regarding the water-bearing properties of 

 the group, gravel will frequently be found lying on 

 the stiff clays and marls of the upper new red sand- 

 stone, and wells sunk therein will supply a good 



1 Stratigraphy is the science of the geological arrangement of Strata. 

 2 Tudsbery ana Brightmore on Waterworks, 3rd ed., p. 124. 



