124 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [J line 17, 



principles, among which the phosphatic matter is held to be the most 

 important ; and hence we may conclude, that the Cornbrash is more 

 fertile than other brashes because it usually contains more of bones, 

 or the principles of bones, diffused throughout the rock ; this in 

 itself would be a sufficient answer to those who would tell us that 

 soils are but little indebted to geological formations for the qualities 

 which they present, — a fact which may be true to a considerable 

 extent in districts where the rock is covered up by local drifts ; 

 but in an elevated district, like the Cotteswolds, in which the high- 

 backed hills present their washed and waterworn sides for farm- 

 operations, we shall find that geological lines can be made out with 

 the greatest possible accuracy by peculiarities both in wild and cul- 

 tivated vegetation ; and indeed we know that the very plantation of 

 dwellings has been guided by similar circumstances, a fact which 

 has been partially dwelt on in the description of the Fuller's Earth, 

 p. 111. 



10. Oxford Clay with Kelloway Rock (9). 



This rock may be first traced on the top of the Cornbrash in a 

 range of low hills a little to the south of Cirencester, as at Sidding- 

 ton, Driffield, and South Cerney ; and these form the advanced 

 guard of a sub-formation, which, still further to the south, takes 

 up a wide range of country, and, in its turn, forms the slightly- 

 raised terrace by which the chalk-hills of Wiltshire are reached. In 

 the line of section, PI. VII., this rock is traced, with the slightest 

 ascertainable dip, to the top of Blunsdon Hill, making altogether a 

 thickness of from 450 to 500 feet ; and here it is overlaid by a mass 

 of Coral-rag : but in the deviation-line (Section, PI. VII.) to Swindon, 

 at C, the Coral-rag appears to be abseut, or to occur only in slight 

 patches, and then the country is taken up with Kimmeridge Clay. 

 Hence, then, the Kimmeridge and Oxford Clays often come toge- 

 ther, so that it is almost impossible in this district to make out their 

 line of demarcation. However, as we shall hereafter see, where 

 either is in force, they can easily be distinguished by well-marked 

 fossils ; and, indeed, even stages of these clays can be made out, 

 not by drawing the line too tightly, and rashly concluding from single 

 fossils, but by a greater or less prevalence, or some peculiar admix- 

 ture, of ascertained forms. 



The following is a general section of the Oxford Clay, as it occurs 

 in its northern extension : — 



feet. 

 60 



3. Beds of dark blue, clunchy clay, very stiff, and highly 1 

 tenacious = the "oak-tree clay" of Smith .... about J 



2. Thick bed of dark-blue or ash-coloured shales 50 



1. Sandy clay beds, with occasional nodules of hard stone 1 ,.^ 

 inclosed in sand and rich in fossils, = Kelloway rock. J 



In these beds the following fossils will, for the most part, be found, 

 in the distribution indicated by the figures, though it must be under- 



