Northampton Oolites. 191 



at the south end of the Isle of Purbeck, in the Vale of 

 Tisbury in Wiltshire, at Swindon, and in the Vale of 

 Aylesbuiy. The beds are very inconstant in their out- 

 crop, only showing at those places which were probably 

 near the original western margin of the sea of the 

 period. At Swindon both limestone and sand are of 

 trifling thickness. Outliers of it occur in Bedfordshire, 

 and the whole has evidently been exposed to denuda- 

 tion before the deposition of the Cretaceous rocks. 



Such is a brief outline of the marine Oolitic strata 

 in the south and centre of England, and also of the 

 Upper and Middle Oolites in their range into York- 

 shire. 



It will be observed that in this description I have 

 specially insisted on the unconformable overlapping of 

 the Cretaceous strata across the Portland, Kimeridge, 

 and other formations, at intervals, all the way from 

 Dorsetshire to Yorkshire, for by-and-by it will appear 

 that this fact has an important bearing on the physical 

 theory of the deposition of the Purbeck and Wealden 

 strata, which come next in succession. 



In the meanwhile, I must return to the Northamp- 

 tonshire area, where we left the Lower Oolites, and 

 follow them into Yorkshire, when it will be seen, that 

 they were formed under physical conditions in some 

 respects very different from those which obtained in the 

 South, while the marine clays and limestones of the 

 Lower Oolites of that area were being deposited. 



It will be remembered that in Gloucestershire, a 

 few miles west of Stow-on-the-Wold, the Fuller's Earth 

 thins out, and the Inferior Oolite and Stonesfield Slate 

 come together, the latter being formed in part of the 

 sandy flags that make the base of the Great Oolite, and 

 constitute the Stonesfield Slate. Going easterly into 



