1857.] BXJCKMAN OOLITES. 119 



ft. in. 



3. Forest Marble sandstones and impure freestone .... 1 6 



4. Clay 1 6 



5. Forest Marble ripple-marked slabs 2 6 



6. Clay to bottom of section 7 



Section at Ampney Cruris, near Cirencester. 



1 . Cornbrash 2 



2. Forest Marble clay 3 



3. Impure freestone with oblique lamination 3 6 



4. Clay 1 6 



5. Impure freestone, sometimes sandy and ripple-marked 3 



6. Clay, with occasionally intervening masses of impure 



freestone 6 



7. Fissile freestone, breaking up obliquely into thin slabs 2 



The fossils of the Forest Marble are mostly identical with those of 

 the Great Oolite, and hence the great difficulty attendant upon a 

 zoological division ; however, there are some so distinctive as to lead 

 to the hope that more extensive examinations of the fauna of this 

 rock would enable the geologist at once to identify it, even where the 

 deposit takes on the oolitic character. 



The first bed of clay, after passing the freestones, introduces us to 

 a peculiarly thin smdjlat species of Oyster*, which is so distinctive, 

 and in such quantity, as to form a good index to the Forest Marble 

 Clay all about this district. 



The slabs of stone are marked by the presence of a Mytilus and 

 Gervillia very distinct from Great Oolite species, and are further 

 distinguished by some peculiarly interesting univalves. 



The peculiarities of the ripple-marked sandstones are such as to 

 render them easily distinguishable ; for, though these are described as 

 much like the Stonesfield Slate, yet the square edge, even surface, 

 and fissile nature of the latter are far different from the rougher 

 aspect of those of the Forest Marble. 



8. Cornbrash. 



This rock, though inconsiderable in thickness, is yet one of the 

 most interesting in the Oolite series. It occupies a wide tract of 

 country in South Gloucester and North Wilts, commencing about a 

 mile from Cirencester, and skirting the Oxford Clay as far as Calne 

 and Chippenham : its position between beds totally distinct in struc- 

 ture, in that it is an oolitic limestone between two thick strata of clay, 

 and the fact that it re-introduces a large number of Inferior Oolite 

 fossils, are remarkable circumstances in the natural history of this 

 deposit. 



It is a bed not more than 15 feet in thickness, and has the litho- 



* May not this extreme thinness of shell be related to the paucity of lime in the 

 argillaceous and siliceous matrix with which they are surrounded ? The marly 

 Bradford Clay contains an Oyster very thick and rugose in its shell, and with 

 which this has been confounded. 



