222 PKUF. S. II. EEY>rOLDS OS THE LITKOLOGlLAL [vol. Ixxvii. 



2-17 per cent, of magnesia, the other of a dolomitized rock 

 containino- 16"7t3 i^er cent, of magnesia. 



Z^ is well exposed in Quarry 1 on the left bank of the Avon, 

 particularly in a series of crags near the middle of the quarrv 

 Avhere the chert-bed is well seen. 



(3j Z.,, (Thickness=about 1S2 feet.) — These beds form the 

 main part of Black Eock Quarry, and are also seen in the riverside 

 exposure. The rocks of this level are extremely uniform in 

 character, and consist throughout of the same general type as the 

 upper part of Z^ — dark dolomitized limestone crowded with large 

 crinoids (petit granit). The well-cleaved calcite of which the 

 crinoidal ossicles are composed catches and reflects the light, and 

 imparts to this rock-type a most characteristic appearance. 

 Foraminifera and CaJcisplicera are plentiful in the greyer and less 

 completely dolomitized bands (A 48, 49) ; but in the prevalent 

 'black rock 'from which the quarry derives its name little has 

 escaped dolomitization, except the large fossils — crinoids, bi-achio- 

 pods, and corals. Dolomitization is very marked in the ' Fish- 

 Beds ' which form the top of Z., and have yielded the numerous 

 teeth and spines of Elasmobranchs. so well known to collectors. 



Z., is splendidly exposed m Quarry 2 on the left bank of the 

 Avon, and the top of Z.,, the whole of y, and the base of 

 the Jaminosa dolomite are seen in the riverside exposure between 

 Quarries 2 and 3. 



Mr. E. B. Wethered^ and Miss M. B. Chapman mention tour- 

 maline and zircon as occurring in the insoluble residue of these 

 limestones. Singularly enough, in vieAv of the widespread dolomi- 

 tization in Z.,. the rocks analysed by Miss Chapman- contained only 

 1'73 per cent, of magnesia, while her example from Horizon y 

 contained 10*6S per cent. 



(4) Horizon y. (Thickness = about 60 feet.) — This level is 

 seen in the southern part of Black Eock Quarry and in the river- 

 side exposure. The same type of massive, dark, crinoidal limestone 

 (petit granit) prevalent in Z.., continues to the top of y. AVhile, 

 as in Z.,. the bulk of the rock is completely dolomitized save for 

 the larger fossils, there are in y some grey bands which are little 

 dolomitized and contain abundant foraminifera and CaJcisplicera 

 (A 50 c/). Patches of calcite, similar to those mentioned above as 

 occui-ring in Z^, occur all through Z.-, and y, particularly in the 

 more highly dolomitized layers. Part of the Caninia-cyJindrica 

 Bed at the top of y presents a patchy appearance, owing to little 

 iiTegular areas of the limestone having to some extent escaped the 

 jDrevalent dolomitization. Horizon y is very well seen in the 

 riverside exposm-e — in fact, better than in the quarry : Zaphrentids, 

 Sijrinrjopora, and crinoids stand out on the weathered surface. 



1 Q. J. G. S. vol. xHv aSSS) p. 190. 

 - Geol. Mag. 1912, p. 500. 



