350 Frof. Sedgwick on the Classification of the Strata [May, 



ginous sand> and a sandstone which is occasionally so coarse as 

 to approach a conglomerate. The oolite at Abbotsbury and 

 some other places is hard and indestructible; more generally it 

 is soft, and liable to decomposition on exposure. The oolitic 

 particles in their size, form, and arrangement, are not so regular 

 as in the great oohte and the Portland stone. Indeed some of 

 these characters are so general that a person well acquainted 

 with the English strata would, in most cases, be able to identify 

 mere hand specimens of the coral-rag oolite. Lastly, the upper 

 beds of the formation are sometimes highly ferruginous, and 

 pass, when the fossils are not abundant, into beds nearly resem- 

 bling those which form the bottom group ; this is the case near 

 Abbotsbury. 



Formation of Corahrag near Oxfords 



Sufficiently minute details respecting this part of the formation 

 are given by Conybeare,* in the " Outlines of the Geology of 

 England ; " and I should have thought it unnecessary to do 

 more than refer to it, had not the arrangement of the groups 

 seemed to contradict that which is given above. It is stated 

 (p. 186), that near Oxford, the calcareo-siliceous grit forms the 

 lowest, the coral rag the middle, and the oohtic freestone the 

 upper part of the deposit. This anomalous arrangement appears 

 to admit of the following explanation. Near Weymouth, and in 

 other-localities, shelly beds (exactly resembling the coral rag of 

 the top group of the general section above given) are often asso- 

 ciated with the middle part of the formation containing the 

 oolite. Where these beds of broken shells, &c. become of con- 

 siderable thickness, they constitute a formation of coral rag 

 alternating with, or inferior to, the oolitic freestone. May not, 

 therefore, the coral rag and superincumbent freestone of Head- 

 ington Hill together represent the central group of the Wey- 

 mouth and Steeple Ashton sections ? The conjecture seems to be 

 confirmed by the appearance of the beds inHeadington quarries. 

 In that place the top freestone supports the Kimmeridge clay ; 

 and the separation between the two is as well defined as a geo- 

 metric line. Now the instantaneous passage from one formation 

 to another frequently indicates the absence of certain beds or 

 deposits.f May not then the upper part of the Weymouth 

 section be wanting near Oxford ? The supposition seems more 

 probable than the inversion of what (upon evidence already 

 stated) appears to be the common arrangement of the subordinate 

 groups. 



• " Outlines of the Geology of England," &c. p. 185—193. In tliis part of the 

 work, Mr. Conybeare assumes the deposit near Oxford as the type of the formation. 



■f The converse of this rule is, I think, true. \VTien a number of deposits succeed 

 each other in a fixed order, and the highest beds of one deposit alternate with the lowest 

 beds of the next supeiior ; the alternation indicates the perfect development of the series. 



