86 PROCEEDINGS OF THE COTTESWOLD CLUB 



completely governs the immediate loss, or, in others, an 

 excess of overflow. 



Drainage areas are not necessarily basins, in the ordinary 

 sense of the term, and lines of water-parting are not 

 always those of greatest elevation. The term river-basin 

 therefore although commonly used and convenient does 

 not apply strictly, except in a few instances. 



Geologically, rivers more frequently run across the dip 

 of the rocks than along their strike. This is a marked feature 

 within that portion of the Thames watershed, occupied by 

 the numerous rivers that pass down the eastern slope of 

 the Cotteswolds, where all the strata from the Lias to the 

 Kimmeridge Clay strike from the south-west to the north- 

 east. The rivers, however, traverse them nearly at right 

 angles, flowing from the north-west to the south-east. 



The geological conditions that affect water supply to 

 rivers are similar over a wide extent of country, when the 

 direction of the axis of the basin corresponds with the line 

 of strike of the rocks ; but the rocks that form the surface 

 at any place may be quite unrecognisable, owing to the 

 presence of a coating or deposit of comparatively modern 

 transported material of which clay, boulders, or rolled 

 fragments of rocks, sand, and gravel, (glacial or river- 

 drift or otherwise) are the chief or constituent parts. 



It is well known that the number and magnitude of 

 streams is much less in districts where the underlying 

 rock is absorbent than in those where it is impermeable 

 and compact. The extreme of this condition is seen in 

 the Chalk, and parts of the Red Sandstone approach it. 



The daily flow of the Thames above Oxford with 

 a drainage of about 600 square miles, and a rainfall of 30 

 inches, varies from about 73,000,000 to 200,000,000 

 gallons, the former being the dry summer weather flow, 

 and the latter the flow during floods, but of which we 

 know little or nothing. 



