INTRODUCTION XXXl 



The Portland Sand exists only as a small outlier, on which the 

 village of Bourton, neai' Shrivenham, is built. Here a section shows 

 the beds of which the Portland Oolite are composed to be about 50 feet 

 thick, the upper bed being a soft, thinly bedded, chalky oolite, with 

 grains of sand, and the lower a hard bluish limestone with pebbles of 

 Lydian stone and white quartz. 



The Lower Green sand consists in the main of sands, often coarse 

 and pebbly, usually more or less rusty in colour, and not unfrequently 

 so strongly impregnated with iron oxide as to deserve the name of an 

 iron ore. It is, however, liable to considerable local variations, the 

 most important of which is the so-called ' gravel ' of Faringdon. Here 

 the formation is almost entirely composed of finely comminuted shells 

 and hard parts of other organisms, in which are embedded fossil 

 sponges, molluscs, brachiopods, and sea urchins in a better state of 

 preservation. Among the characteristic fossils from the Faringdon 

 (xravels, figured in Phillips' Geology of Oxford, are Manon Faringdonensis, 

 Cidaris Faringdonensi^, and Lima Faringdonensis. 



The outcrop of the Lower Greensand does not stretch, like the out- 

 crops of the other formations, in an unbroken belt across the county. 

 From the river Cole in the west it runs as a narrow strip to Faringdon, 

 and then spreads out into a broad patch. Very little further to the 

 east it is overlapped by the Gault, and it does not show at the surface 

 till the neighbourhood of Culham on the Oxfordshire side of the 

 Thames. Outliers of it occur on Boar's Hill, where it reaches its 

 loftiest altitude in the county on Pickett's Heath, which is 535 feet 

 above sea level, and on Cumnor Hurst. The formation also peeps out at 

 Clifton Hampden, and is seen in some bold picturesque cliffs on the 

 Oxfordshire side of the Thames. In the west of the county, Faringdon 

 Clumps and Badbury Hill, both about 500 feet high, are capped with 

 the Lower Greensand ; in the latter place it is more or less fossiliferous ; 

 some wood and a fern leaf have been found in the sandstone which 

 caps the whole. 



It must also be noticed that, while the various subdivisions of the 

 Jurassic System rest upon one another in the same order, this is no 

 longer the case when we pass to the overlying Neocomian Beds, for in 

 the neighbourhood of Faringdon the Lower Greensand lies sometimes 

 on Corallian Beds. The meaning of this is, that between the Jurassic 

 and Neocomian Periods there intervened a time during which the 

 Jurassic Beds were uplifted, tilted, and denuded, and that the 

 Neocomian Beds were not laid down till these operations had been 

 completed. In geological terminology the two systems are uncon- 

 formable to one anothei'. Similarly the overlap of the Gault on the 

 Lower Greensand shows that the Cretaceous Beds rest unconformably 

 on the Neocomian. 



