THE JURASSIC SYSTEM 449 



The Purbrrk Beds are cut off and faulted against the Chalk and 

 (Jreensand by the great Ilidgeway fault, and they are not seen 

 again till they crop out in the Vale of Wardour (Wilts). The 

 ion in tliis area, as described by Mr. W. R. Andrews and 

 inysi-lf in 1894, - 1 with such subsequent corrections by Mr. H. B. 

 Woodward as we accept, is as follows : 



Feet. 

 ( Clays, marls, and shelly limestones with Cypridea 



Upper -j punctuta, Cyrena media, and Unio . . . .10 



| Yellow sand and grey clay 12 



( Sandy and marly limestones, shelly marls, and cinder 



Middle -! bed with Ostrea distorta and Trigonia gibbosa . . 12 

 (Limestones and shelly marls, Cypridea granulosa . . 11 

 /Marly limestones (locally known as Lias) with layers 



T- . | of shaly marl, Cypris purbeckensis .... 20 

 1 Pale yellow oolitic limestones and marls ... 25 

 I Grey "laminated marls and limestones .... 20 



Total about 110 

 c d e f 



V\X. I'lO. DIAGRAMMATIC VIEW OF THE SWINDON QUARRIES (after PlX>feS8Or J. F. Blake). 



;/, /i, i. Purbeck Beds. b. Sand with calcareous doggers 



. / Portland limestones. a. Portland limestone. 



The Lower Purbeck Beds are seen again at Swindon, where 

 they rest partly on the Upper Portland and partly on the Portland 

 sands (see Fig. 150), and their basement bed contains rolled lumj- 

 of stone derived from the Portland Beds. 



In the Midlands the Purbeck Beds only emerge from beneath 

 the Cretaceous strata in Oxford and Bucks. Around Thame, 

 Aylesbury, Brill, and Whitchurch the Portland Beds are succeeded 

 by a variable group of beds with freshwater fossils, which are 

 generally considered to be of Purbeck age. They consist of a variable 

 series of thin-'bedded limestones, marls, shales, clays, and calcareous 

 sands, but do not exceed 30 feet in thickness, and probably represent 

 the Lower Purbeck only, the higher beds having been removed by 

 Cretaceous erosion. They have yielded remains of Plants, Insects, 

 and Cyprides, with Viviparus, Cyrena, Modiola, and the fish 

 Lepidotus minor, Pleuropholis serrata, Aspidorhynchus, and Mesodon. 



2o 



