CHAP. IX.] JURASSIC PERIOD. 143 



but it is clear from this remarkable section that the upper 

 beds here were formed in much shallower water, and that 

 the area was rapidly converted into an estuary, or fresh- 

 water lake. When next seen in Oxford and Bucks other 

 changes are found to take place ; the lower sands thin out 

 gradually from 70 feet at Shotover to 6 near Aylesbury, 

 and the upper beds consist of limestone and fine silty sand 

 in alternate layers (25 feet), passing up into a set of marly 

 shales and limestones of freshwater origin (15 feet) ; these 

 last, though homotaxially the equivalents of the Purbeck 

 beds, are probably, in reality, of Upper Portland age. Beds 

 of this character occur within a few miles of Leighton 

 Buzzard, and may originally have stretched a little further 

 north-east, as Portlandian fossils occur in the Cretaceous 

 sands of Bedfordshire; but no Portlandian strata are 

 again found till we reach Yorkshire, where black shales of 

 this age overlie the Kimeridge Clay at Speeton. 



As regards their south-eastern extension, sands and sand- 

 stone (about 80 feet thick) occurred at this horizon in the 

 Sussex boring, and they are found in the north of France, 

 so that they are doubtless continuous beneath the Channel, 

 but they do not appear at the western outcrop of the series 

 in Normandy. 



Before noticing the Purbeck Beds we may glance at the 

 correlatives of the marine Upper Jurassic series in Scotland. 

 On the west coast the Oxford Clay only is seen, but in 

 Sutherland a more complete series is found : the Oxford Clay 

 is over 300 feet thick, and is marine throughout ; it is suc- 

 ceeded by a zone of white cherty sandstone, with the fossils 

 of the Lower Calcareous Grit overlain by a mass of estua- 

 rine sandstones with layers of lignite ; these are 400 feet 

 thick, and are surmounted by marine beds with Coral Eag 

 fossils. The Kimeridge Clay is also represented by a thick 

 and variable series of beds, chiefly estuarine sandstones in 

 the lower part, with marine shales and limestones (500 feet 



