318 Revieu-H — The Geological Surreij — 



of the Oxford Clay are here represented." If that can take place 

 within 40 feet of beds, of what value are these particular ammonites 

 for zoning 400 feet of beds ? 



The Corallian beds within the area receive their share of attention, 

 and, being for the most part accessible and fairly fossiliferous in places, 

 their history is not difficult of interpretation. Measured on the scale 

 it would seem that the average development does not exceed 50 feet, 

 including the Calcareous Grit at the base with the Coral Rag and 

 Coralline Oolite at the top. In the explanatory column a wedge of 

 Ampthill Clay is driven into the Corallian Series, which as a calcareous 

 body disappears a little to the north-east of Oxford. The chartographer 

 has also ventured to represent a considerable extent of Ampthill Clay 

 in this quarter, separating the Oxford from the Kimeridge Clay. The 

 adjacent Arngrove stone by its fauna seems to represent the Calcareous 

 Grit. It is in this direction perhaps that the greatest change in the 

 solid geology of the map occurs ; since a very considerable area in 

 the north-east corner formerly coloured as Lower Calcareous Grit, 

 in the region between Boarstall and Waterstock, now appears as 

 Ampthill Clay. 



The Kimeridge Clay of the district probably nowhere exceeds 

 100 feet, a remarkable falling off from its development on the Dorset 

 coast. One is tempted to speculate as to how far the Kimeridge 

 Clay of Oxfordshire is the exact equivalent in time of the Kimeridge 

 Clay of Dorset. Certainly there is no passage to be seen between the 

 Kimeridge Clay and the Corallian, such as may be well observed in 

 Eingstead Bay. The question then arises as to how far the basal beds 

 of the Kimeridge Clay are developed in Oxfordshire. Good specimens 

 of Rhynchonella inconstans are quoted from Sandford, and this fossil is 

 characteristic of the lowest beds in Dorset ; moreover, Sowerby figured 

 Ostrca deltoidea from Headington, and this is also indicative of a low 

 horizon, though not to be relied upon like the other fossil. Phillips 

 is stated to have observed in the section on Shotover Hill, about 15 feet 

 from the base of the Kimeridge Clay, a septarian band yielding 

 Rh. inconstans, etc., whilst lower down there are layers of Ostrea 

 deltoidea, and near the base Thracia di'pressa and JExogyra virgula are 

 fairly abundant. Hence the relative position of these fossils is the 

 reverse of that in Dorset. But there are exceptional features in 

 the fauna of this region, for we note that Perisphinctes plicatilis is 

 quoted from quite the top of the Kimeridge Clay in a boring at 

 Baldon, and the same species of ammonite is quoted with a query 

 from the Lower Portlandian of Shotover. 



These considerations, however, do not mateiially affect the mapping, 

 which must proceed mainly on lithological lines. Hence there is not 

 much difficulty in determining the several Portlandian outliers which 

 give such variety to the south-east corner of the map. In this 

 connection, also, we observe that a considerable portion of the 

 district, comprising the Baldons, formerly mapped as Lower Greensand, 

 is now added to the Lower Portlandian. The thickness of the Port- 

 landian at Shotover is estimated at rather over 100 feet. "This 

 group," Mr. Woodward observes, "consists mainly of sandy strata 

 with beds of limestone, as well as sand and thin clay-bands in the 



