426 THE CRETACEOUS SYSTEM. CHAP. 



hills, or between them and Faringdon. On the contrary, the gault 

 clay and upper greensand of that cretaceous range are found to 

 rest on lower strata, viz. on Kimmeridge clay. Thus we have 

 two cases of unconformity in this region between the Thames 

 valley and the Wiltshire downs. (See Plate III.) 



i. The lower greensand rests unconformably on the Kim- 

 meridge clay, the lower part near Faringdon, no Portland 

 being found between these strata. 



1. The gault rests unconformably on the Kimmeridge clay 

 (the upper part) south of Swindon, neither Portland nor 

 lower greensand being observable between them. 



These deficiencies of the Portland rock and lower greensand are 

 clearly due to wasting of those deposits, such waste beginning 

 after the Kimmeridge sera, and continuing at intervals till after 

 the age of lower greensand. No wonder if, under such conditions, 

 here and there Wealden beds of fluviatile origin as to the materials 

 should be locally interpolated in the series of marine sediments. 



We may now proceed to the country south of Oxford, where 

 a considerable extent of surface is occupied by the lower greensand, 

 though greatly disguised by an overspread of pleistocene drift. 



Nuneham Park gives a succession of gault, lower greensand, 

 and Kimmeridge clay ; the sandy strata are somewhat ferruginous, 

 coarse-grained, and pebbly. The stratum is really continuous to 

 Culham on the south-west, Toot-Baldon, Marsh - Baldon, and 

 Chiselhampton on the north-east and east. It appears in cliffs 

 at Clifton- Hampden, above the Thames, and is found in the channel 

 of that river. Here it is a strong ferruginous conglomerate, with- 

 out fossils. At Toot-Baldon it caps a hill of Kimmeridge clay ; 

 and in its ferruginous beds some fossils have been found by myself, 

 and also by the members of the Ordnance Survey. Ammonites 

 Deshaysii is one of these. 



One of the most interesting of the sections near Oxford is seen 

 at Culham, on the northern bank of the Thames; and this may be 

 compared with another in the line of railway near Culham Station, 

 about a mile to the north-east, with a hill-capping at Toot-Baldon 

 and a cliff-section at Clifden. I have been in the habit of taking 

 my class to some of these localities for several years. 



On entering the excavation at Culham we perceive about 40 feet 

 of level-surfaced clays and sands, under a cover of flint-gravel mixed 



