Oolitic Formations in the vicinity of Minchinhampton, 249 



Ckemnitzia ; the bivalves are again of full dimensions, and the 

 Cephalopoda reappear in full force and of large size, but being 

 difficult to extract entire, are rarely seen in the cabinets of col- 

 lectors. But to form an idea of their numbers, the lowest bed in 

 the escarpment at Frocester Hill should be visited ; it is a perfect 

 storehouse of this class of remains. Nor is this abundance con- 

 fined to one locality; wherever the brown ochrey beds are ex- 

 posed in the escarpments of our valleys, or on the outer line of 

 the Cotteswolds, a single square yard of rock exposed is usually 

 sufficient to produce fragments of Ammonites and Belemnites ; 

 and it would appear that a similar profusion of those forms di- 

 stinguishes the lower beds of Inferior Oolite throughout the 

 whole of its course in Somerset and Dorset. That they should 

 entirely cease between Painswick Hill and Crickley Hill, to be 

 replaced by other and totally different beds of rock and fossil 

 contents, is one of the most interesting geological problems which 

 the Cotteswolds offer to the scientific inquirer. A very remark- 

 able Brachiopod marks the base of the formation in our district ; 

 Terebratula bidens occurs in the lower rag- stone, and more espe- 

 cially in a few inches of marly rock, sometimes called Gingerbread 

 rock, which immediately underlies it. Terebratula acuta and 

 T. tridens accompany it much more sparingly ; the latter possibly 

 may be only a variety of the first : the separation of species among 

 the Brachiopoda must be regarded as merely provisional until 

 the state of our knowledge respecting them shall be more ad- 

 vanced. The brown sands beneath are entirely barren of organic 

 relics, and gradually and insensibly merge into the Upper Lias. 



The general diminutive appearance which the Great Oolite shells 

 present when compared with those of the other oolitic rocks cannot 

 fail to be noticed. In species which have a considerable vertical 

 range this fact is rendered particularly striking : thus, but for a 

 perfect identity of markings, Trigonia costata reduced to the size 

 of a bean, and sometimes even of a pea, would scarcely be re- 

 garded as the representative of the large Inferior Oolite shell : 

 higher in the Oolites it again attains its pristine dimensions. 

 Astarte excavata too, without the aid of a large series for com- 

 parison, would not be recognised ; the shell becomes small, de- 

 pressed, and the costse rendered almost obsolete. Modiola pli- 

 cattty which reappears in the upper beds of Great Oolite, nearly 

 loses its plicae, and acquires a compressed angular form. The 

 changes of size which Lucina lyrala undergoes is still more re- 

 markable. In the lower rag-stone it is of full dimensions ; in the 

 Nerinea bed or Fimbria bed it is reduced to one-fourth its former 

 bulk ; in the upper rag-stone it is again large ; in the shelly beds of 

 the Great Oolite it is rare, but is again reduced to the dimensions 



Ann. ^ Mag. N. Hist. Ser. 2. Vol. ii. 18 



