CHAPTEK XVI. 



THE CRETACEOUS SYSTEM. 



THOUGH not fully exhibited in the district round Oxford, there 

 are traces of all the parts of this great system, and much diversity 

 in some of the members of it, especially the glauconitic parts 

 upper and lower greensand. 



Upper chalk, with bands and nodules of flint. 

 Middle chalk, including the * chalk rock ' of Whitaker. 

 Lower chalk, with marly partings, and no flints. 



Upper greensand, or fine sandy chalk. 



Gault, pale blue laminated clay. 



Lower greensand, often ferruginous and pebbly. 



LOWER GREENSAND AND CONGLOMERATE. 



The passage from the oolitic to the cretaceous system of strata 

 is nowhere in the British islands very gradual, either in respect of 

 concordant deposition, mineral composition, or organic life. The 

 estuarine and fresh-water conditions manifested at intervals in the 

 oolitic series, and especially abundant in the upper part, may be 

 supposed to have continued in the Wealden, while elsewhere the 

 neocomian beds of marine origin had contemporaneously entered 

 an appearance. Confining our attention to the ample range of 

 country accessible from Oxford, between the districts of Devizes 

 and Cambridge, we find almost everywhere the gault clay in fair 

 antecedence to the upper greensand, which is not always clearly 

 separable from lower chalk and upper chalk. But the beds below 

 the gault are just as often Kimmeridge clay in the Vale of North 

 Wilts, or Portland rock in the Vale of Aylesbury, as lower green- 

 sand, so prevalent is the unconformity between the truly cretaceous 



