CHAPTER XIII. 



THE PORTLAND OOLITE PERIOD. 



THIS series of oolite, sand, and clay is nowhere in England con- 

 tinuous for great spaces, except in its lower portion. The order 

 of succession in the Oxford district is found to be thus : 



Sands with nodular concretions. 

 Rough calcareous beds. 

 Sands with nodular concretions. 

 Kimmeridge clay. 



In Smith's f Table of British Strata' the series is presented in 

 the same form. In Shotover Hill the limestone beds are very 

 incomplete, and indeed hardly separable from the large nodular 

 concretions full of fossils which are so remarkable there. At 

 Swindon the calcareous portion is, on the contrary, much more 

 considerable, but hardly oolitic in texture. 



KIMMERIDGE CLAY. 



From the great cliffs of Kimmeridge in the Isle of Purbeck, 

 which give name to the deposit, this clay is traceable (with no 

 interruptions except such as arise from the unconformed over- 

 extension of the chalk) into Yorkshire, everywhere preserving its 

 characters of nearly uniform composition, great thickness, and 

 peculiar organic fossils. The upper part is observed to be greenish 

 in the vicinity of Oxford, and sandy, the tint being derived from 

 glauconite, the common ingredient of greensand. In the railway 

 cutting of Shotover Hill sandstones of a greenish tint interlaminated 

 the upper parts of the clay, and announced the proximity of the 

 next deposit by containing ammonites, pholadomya, and other fossils 

 of the Portland group above. 



