EXPLANATION OF THE HYDRO-GEOLOGICAL MAP 



The accompanying map of the Upper Basin of the Thames, above 

 WalHngford, or the area occupied by the Jurassic Strata between the 

 northern edge of the map, and the strike of the Lias on the N. and 

 N.W., and the parallel outcrop of the Chalk between WalHngford on 

 the E., and S. of Swindon on the W., embraces nearly the entire area 

 occupied by the Lias, Inferior and Great Oolite, Forest Marble, 

 Cornbrash, Oxford Clay, Corallian Beds, Kimmeridge Clay, and 

 Portland Oolite — or Jurassic group of Rocks. 



These are again succeeded by the Cretaceous series, ranging through 

 the Lower Greensand, Gault, Upper Greensand, and Chalk. 



A sectional line drawn from the N.W. to the S.E. across the 

 Cotteswold Hills from Cheltenham, through Syreford, Northleach, 

 Burford, Witnev, Wytham Hill and Oxford to Great Milton, passes 

 nearly at right angles the outcrop of the several sub-divisions of the 

 Jurassic series. 



From Great Milton the Cretaceous group are also crossed by the 

 same continuous line. Thus no less than 12 well-defined and 

 characteristic divisions of the Oolitic series are exposed, and 5 

 divisions of the overlying unconformable and succeeding Cretaceous 

 series above named, at WalHngford the extended range of Chalk 

 indicates the position or commencement of the middle grouping of the 

 Thames Basin. 



These Jurassic and Cretaceous Rocks have been severally noticed 

 in my paper. The rivers which have traversed them, both those that 

 cross the strike of the several permeable and impermeable groups of 

 strata and those that run with or parallel to or with them. 



The position or sites of the more prolific springs are shown by dots 

 at their issue ; these springs and the courses taken by the streams are 

 an important feature in the Hydro-Geology of the Cotteswolds, 

 especially those that have their issue and course through the Oolitic 

 Rocks N. of the Thames. 



The Cretaceous Springs, although numerous, are less prolific and 

 more uncertain in their supply, and chiefly enter the Thames from 

 the S. 



Probably 200,000,000 gallons per day passes over the Bensington 

 Weir above WalHngford, where the whole of the water supplied by 

 the springs from the S.E. slopes of the Cotteswold Range, and those 

 passing from the higher Jurassic and Cretaceous Rocks defined by the 

 Chalk outcrop or water-parting can be gauged. 



