84 PROCEEDINGS OF THE COTTESWOLD CLUB 



upon or between (in the case of the Upper Thames 

 district) the Lias and Fuller's Earth Clays, and its ultimate 

 delivery through springs to the streams and rivers, is or 

 may be spread over weeks and months. 



To this cause we owe the permanence and regularity of 

 the flow of the Thames, thus draining the permeable 

 rocks, both of the Upper and Middle Basins ; the 

 former only however being here under description. As 

 compared with the irregular delivery from an imperme- 

 able area this is a consideration of great importance in the 

 question of water supply, and it is well to notice the value 

 of the impervious rocks, as affording copious supplies of 

 water to the Thames, which at once receives them either 

 as flood or otherwise. The following seven groups of 

 strata show that the large proportion of rain falling upon 

 their surfaces is not absorbed but runs off towards the 

 river : 

 From (i) the Lias — A great portion passes off the surface, 



except where covered with alluvial or gravelly 



soil. 



(2) Fuller's Earth — Where exposed nearly all runs 

 off, but the area is usually very small. 



(3) Forest Marble — Part absorbed by sandy beds, 

 but owing to bands of clay, about ^ of the 

 remainder runs away. 



(4) Oxford Clay .and (5) Kellaways Rocks — Part 

 runs off and part is absorbed, but the whole of 

 the water of the Oxford Clay passes off and 

 reaches the Thames. 



„ (6) Kimmeridge Clav — A considerable portion runs 



off. 



(7) Gault — The greater portion runs off, but is 



partly absorbed again by the Lower Greensand. 



The proportion of rainfall that may be expected to be 



absorbed or run off from a given drainage area, the 



consequent supply yielded to the rivers, the quality of the 



1 



