82 PROCEEDINGS OF THE COTTESWOLD CLUB 



east of Oxford. This Argillaceous Zone occupies an un- 

 broken area of nearly 420 square miles, averaging about 

 eight miles in width by fifty, between the extremes of 

 the south-west and north-east sides of the watershed 

 and is probably 150 feet thick. The CoraUian Oolites are 

 parallel to, and occur between the Oxford and Kimmeridge 

 Clays, but more or less interrupted along their strike. 

 Swindon, Highworth, Faringdon, Thame, and Aylesbury 

 are placed upon these uppermost members, which ter- 

 minate the Calcareous series, or second Zone, of the 

 Jurassic Rocks within the watershed. These beds may be 

 between 60 and 70 feet thick. The third Impervious 

 Zone, parallel to the CoraUian, comprises the Kimmeridge 

 Clay, which cannot be less than 300 feet thick, occupying 

 about 220 square miles, between the extremes of the 

 western and eastern sides of the watershed. The south- 

 ern outcrop of the Kimmeridge Clay terminates the visible 

 mass of the Upper Thames Jurassic Rocks, so definitely 

 marked by the six parallel groups of strata occupying the 

 elevated tract of country nearly 1200 square miles in 

 extent. These six groups are the Lias, Inferior Oolite, 

 Great Oolite, Oxford Clay, CoraUian Oolites, and Kimmer- 

 idge Clay. 



The Jurassic Rocks are succeeded by the unconformable 

 yet parallel outcrop of the Cretaceous series, or the lower 

 Greensand and Gault ; the former being about 400 feet 

 thick, and the Gault from 130 to 150 feet; averaging 3 

 miles in width by 56 from W to E, or 168 square miles. 



The fourth Impervious Zone, or the Gault, ranges 

 south-west and south-east from Wroughton, Wanborough, 

 Wantage, Wallingford, Princes-Risborough and Wendover 

 to Irvinghoe, between the western and eastern sides of the 

 watershed. This strike also defines the northern outcrop 

 of the Chalk basin, the exposed area of which between 

 the above line and the parallel strike of the Lower Eocene 

 series, from Hungerford on the south-west to Reading, 



