CHAP. IX.] JURASSIC PERIOD. 141 



shales, sandstones, and limestones of marine origin, and 

 380 feet thick, but including 60 feet of white sandstone, 

 with bands of shale containing plant remains. But of the 

 Great Oolite there is no marine representative, its place 

 being taken by a remarkable formation which is evidently 

 part of the delta of a large river ; the mass of this consists 

 of grey and white sandstones often current-bedded and 

 ripple-marked, but both above and below these are groups 

 of black shale and limestones, in which freshwater shells 

 are abundant, together with the remains of reptiles, turtles,, 

 and fish. Professor Judd remarks upon the striking re- 

 semblance which these beds present to the Purbeck series of 

 Dorset. Similar beds occur on the east coast of Sutherland, 

 where the highest member of the series is a coal-seam 

 3f feet thick, resting on black shales with plants and 

 crushed freshwater shells. From these facts it is clear that 

 at this epoch the Scottish gulf was entirely silted up, and 

 converted into swampy land, like that composing the delta 

 of any large modern river. 



Upper Jurassic Series. Like the Lias this is essentially 

 an argillaceous series, but it includes large lenticular and 

 episodal developments of limestone and calcareous sand. 

 The succession is most complete in the south of England, 

 where it consists of the following members in descending 

 order : 



5. Purbeck beds, estuarine and freshwater. 



4. Portland limestone and sands. 



3. Kimeridge clay. 



2. Corallian limestone and sands. 



1. Oxford clay. 



The Oxford Clay is generally more or less sandy at its. 

 base, but the mass of the formation consists of dark grey 

 or blue clay with layers of calcareous nodules. In the south 

 of England it is from 300 to 600 feet thick, and it main- 

 tains the same character and average thickness to the north 



