XVI. 



CULHAM SECTION. 



427 



with worn shells of Gryphsea dilatata and other spolia of the 

 adjacent country. Nearly the whole mass of the clays and sands 

 excavated here is employed for brick-making; and the digging 

 operations mix them much together. A slight glance at the section 

 presents enough of uniformity to induce the belief that the whole 

 might belong to one continuous deposit. If, under this impression, 

 a palaeontologist viewing the excavation should pick up Thracia 

 depressa and Cardium striatulum, and obtain from the workmen 

 teeth of pleiosaurus, he will probably write ' Kimmeridge clay' on 

 the whole section. Another geologist, arriving when the clay is 

 not being dug, may examine a different part of the deposit and find 

 Ammonites dentatus and Belemnites minimus, and may colour 

 on his map, ' undoubted gault.' But when, instructed by several 

 visits, the whole section is clearly made out, we find two clays 

 in the pit, of entirely different geological age, separated by a bed 

 of sand apparently conformed to each so far as this very limited 

 area gives any evidence. 



Ammonites dentatus 

 Nodules 



Sandy and gravelly partings . . 

 Basement bed, pebbly and fer- 



ruginous 



Fine-grained sand, nearly uni- 

 form in composition , 



Zone of fossils : Ammonites . . 



Brown nodules, with crystals 



(Bisulph. zinc) in the cracks. 



Ammonites 



Mixed gravel of Culham Fields, 

 containing northern drift. 



Gault : blue laminated clay full 

 of fossils (20 feet seen). 



Lower greensand. 

 Green sandy cap of the Kim- 

 meridge clay (9 feet). 



Kimmeridge clay (23 feet seen). 



Diagram CXCVI. Section of the Strata at Culham, south of Oxford. 



