120 



PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 17, 



logical structure of an Inferior Oolite ragstone, being similar in 

 structure and chemical composition to the Gryphite and Trigonia grits 

 of the Lower Oolite rock ; this will be best explained by the follow- 

 ing Table of Analyses of my friend and coadjutor Professor Voelcker*. 



Analyses of Oolite-brashes, by Professor A. Voelcker. 



Carbonate of lime 



Magnesia o.... 



Sulphate of lime 



Alumina 



Phosphoric acid 



Soluble silica 



Insoluble siliceous matter . 

 Alkaline salts, undetermined. 



Inferior 

 Oolite. 



Great 

 Oolite. 



89-20 



•34 



•09 



414 



•06 



2-75 



3-27 



99-85 



95-346 

 •739 

 •204 



1-422 

 •124 



1-016 

 •533 



99-384 



Cornbrash. 



89-195 



•771 



•241 



2-978 



•177 



1-231 



4-827 



99-420 



It will be seen from this Table that the carbonate of lime is of 

 equal amount with that of the Inferior Oolite ; and hence, in as far as 

 composition is concerned, we have in the Cornbrash a limestone-bed 

 occurring in the midst of others of quite different character ; and the 

 result of a recurrence to the same physical conditions has been a 

 return to an Inferior Oolite fauna f, even after a lapse of time suffi- 

 cient to have formed as much as 300 feet of rock of diverse litholo- 

 gical structure, each rock containing its own set of fossils. 



Our analytical Table also shows us a marked difference in the 

 quantity of phosphoric acid which different oolite-brashes contain ; 

 and, as the term Cornbrash is derived from its agricultural capabili- 

 ties, these with other considerations seem to demand a consideration 

 of this deposit under the three following heads :— 



1st. Its position and structure, 



2ndly. Its fossil contents, and 



3rdly. Its agricultural peculiarities. 



1 . Thin as is the Cornbrash, it occupies a considerable horizon- 

 tal extent ; this is accounted for from the general flatness of the 

 country to the south and east of Cirencester, as also from the slight 

 amount of dip in this direction. 



About two miles on the Cricklade Road in the line of section (PL 

 VII.) is a quarry which gives the following section, and has yielded 

 a large proportion of the fossils hereafter to be tabulated. 



Corn- 

 brash 



Section on Cricklade Road. 

 ■ 11. Rubbly incoherent stone, full of shells .... 

 . \ 2. Hard ragstone, with uneven splintery fracture 

 3. Forest Marble clay ; bottom of quarry. 



ft. in. 



2 

 8 



* See Proceedings of the Cottesvvolds Nat. Club, vol. i. p. 263, &c. 



t It is remarkable that every fossil figured by Phillips in his ' Geology of York- 

 shire ' illustrative of Cornbrash equally well illustrates the Inferior Oolite of Glou- 

 cestershire. 



