199 



Bradford Clay. — In the Minchinhampton district the soft 



upper beds of the Great Oolite are succeeded by a bed of very 



hard, white, or cream-coloured Limestone, eagerly sought out 



for the kiln, and raised for that purpose ; this rock is remarkable 



for its hardness and the closeness of its texture. It is well 



seen in many places, as at Bussage, Cowcombe, near Cirencester, 



and in the open cutting of the Sapperton Tunnel ; at Bussage 



and Cowcombe it contains a great many remarkable shells, 



the tests of which are preserved in the form of highly 



crystallized carbonate of lime, such as Pachyrisma cjrande, 



Natica Michelini, N. grandis, and Purpura nodulata ; the hard 



white Limestone is considered as the uppermost member of the 



Great Oolite in this district. At the Tetbury Eoad Station of 



the Great Western Eailway, a bed of yellowish clay is exposed, 



containing Terebratula digona, T. orbicularis, T. maxillata, T. 



coarctata, T. cardium, Avicula echinata, Pecten hemicostatus, 



Cidaris Bradfordensis, Pseudodiadema homostigma. These forms 



characterize the clay above the Great Oolite in Wiltshire, 



known as the Bradford clay ; this rock is not a persistent 



stratum, but is frequently wanting, as near Bradford, where the 



Forest marble being visible is seen resting on the Great Oolite 



at Pickwick and Wormwood.* The Bradford clay ought not 



to be considered as an independant formation, separating the 



Great Oolite from the Forest marble, but rather as one of the 



inconstant argillaceous bands of the latter formation; this clay, 



above the upper Oolite, sometimes attains a thickness of from 



40 to 60 feet at Farleigh ; in other localities it is much thinner. 



Near Bradford, Wilts, it contained a great number of fine 



specimens of Crinoids, Apiocrinus ParJcinsoni, and numerous 



Brachiopoda, Terebratula digona, T. coarctata, T. cardium, &e. 



When the Bradford clay is wanting, it is almost impossible to 



distinguish the upper beds of the Great Oolite, from those of 



the next formation. 



Forest Marble. — This formation consists of a coarse fissile 

 Oolite full of false bedding, and having interstratified therewith 



* Lonsdale, the Oolitic district of Bath, Trans. Geol. Soc, 2nd Series. Vol. 

 iii., p. 255. 



