The Geology around Oxford. 317 



with the exception of the Oxford Clay, of whose original outcrop nearly 

 one-half is masked by the alluvium and gravels of the Thames River 

 system. Large collections of Jurassic fossils from the immediate 

 neighbourhood enrich the Oxford Museum, but the Corallian rocks 

 cannot vie in development and palaeontological wealth with those of 

 Dorset and JS'orth-East Yorkshire, neither do the scattered Portlandians 

 in the south-east corner yield such a suite of fossils as may be found 

 in Wilts and Bucks. 



In dealing with the solid geology of the area a very comprehensive 

 sequence of the Jurassic rocks is shown, whilst boring operations have 

 yielded some interesting details respecting those beds which do not 

 actually appear on the surface within the limits of the map. Thus, at 

 Fawler the Inferior Oolite has already shrunk from the fine display on 

 the Cotteswold escarpment to a thickness of 37 feet, whilst underneath 

 Oxford the thickness of the Inferior Oolite is only 16 feet, and we 

 may strongly suspect that it thins out to a feather-edge a little farther 

 to the eastward. The easterly attenuation of the Lias is equally 

 remarkable. Its thickness in the Northern Cotteswolds was proved 

 to be 1,361 feet, and of this the Upper and Middle Lias account for 

 400 feet. Yet these two subdivisions are held to be absent under 

 Oxford, where the actual thickness of the Lower Lias is at present 

 unknown. These facts may be further confirmed from a study of 

 the east and west section which accompanies the map, where both 

 the chief borings about Oxford are represented as terminating in the 

 Lower Lias, whose thickness at Burford was found to be 447 feet. 



Bathonian beds, consisting of Great Oolite, Forest Marble, and Corn- 

 brash, come to the surface in the extreme north-west corner of the 

 map. It is probable that the Great Oolite, as a limestone series, 

 attains its maximum development in Oxfordshire and the adjacent parts 

 of Gloucestershire. With the Cornbrash the Lower Oolites are held 

 to terminate, and on this rests the chief feature of the map, viz. the 

 Oxford Clay with its Callovian base. The arenaceous features of this 

 horizon must be feebly developed near Oxford ; thus the rule that clays 

 predominate hereabouts in the Middle Oolites may be said to prevail. 

 Hence there is no special sandstone coloured in the map or column of 

 explanation, merely a reference to " sand and clay " towards the base. 

 On the scale attached to the map the thickness of the ' ' Oxford Clay 

 and Kellaways Beds " works out at about 450 feet, whilst on p. 23 

 the authors speak of the Oxford Clay as being " nearly 400 feet thick 

 in this region." It is difiScult to understand how such can be the case 

 when the well at the city brewery only shows 210 feet of Oxford 

 Clay down to the Cornbrash. There may be two reasons for this 

 excessive estimate : first, the notion originally derived from Phillips' 

 interpretation of the Wytham boring, which was corrected by Prestwich ; 

 second, the extraordinary thickness of the Oxford-Kellaways formation 

 at Swindon, estimated at 572 feet. There is also another circumstance 

 in connection with the Oxford Clay of this district which excites 

 surprise, viz., that within the space of a 40 feet section in the 

 Summertown pit Cardioceras cordatum occurs in the upper part, lower 

 down there is Cosmoceras Buncani, and likewise Keppleriies calloviensis, 

 together with M. macrocephalus. " Thus portions of the chief zones 



