part 3 J SUCCESSION of the avoniajst at clteton. 221 



It shifts the outcrop of Horizon j3. That horizon is well exposed 

 at the northern end of Quarry 1 on the left bank of the Avon. 

 The lowest beds, which consist of reddish thinly-bedded limestone, 

 form the dip-slope at the northern end of Quarry 1, and are highly 

 fossiliferous. 



(2) Z^. (Thickness =134 feet.) — The beds that form the massive 

 wall overlooking Press's Quarry are the base of Z^. They are 

 rather thinly-bedded, and consist mainly of somewhat coarsely 

 crinoidal limestone ; sometimes rhombs of dolomite are plentifully 

 distributed in the matrix among the crinoidal fragments. Some 

 bands are laminated, with wisps of shale interbedded in the finely- 

 divided crinoidal material. Much of the lower part of Z, is, 

 however, strongly dolomitized, and occasionally shows a cavernous 

 structure on the joint-faces. In one band near the base the 

 dolomitization has a peculiar patchy character, and the sharply- 

 defined boundaries of the dolomitized areas suggest infilling or 

 hrocciation. Some of the bedding-planes which form a prominent 

 feature at this horizon are determined by narrow shaly partings 

 crowded with brachiopods, while others occur at the level of bands 

 of compact pale dolomite. Bryozoa are rather plentiful in some 

 layers (A 41). 



At a level about half-way up Z^ there is a band of chert (A 46) 

 due to replacement of the limestone. Sections show that silici- 

 tication is not complete, rhombs of dolomite being scattered through 

 the chert. No sign of sponge-spicules or radiolaria was seen. 

 The crinoidal ossicles, as is usually the case, show themselves very 

 resistant to the process of silicification. A certain amount of 

 silicification was observed in the Z^ fossils, both above and below 

 the chert, but less than is the case in most of the local sections 

 of Z^. 



Vaughan took the prominent bedding-plane seen where the path 

 leading to the opening for the tunnel through the Downs bears 

 off, to be at the upper limit of Z^. Upper Z^ chiefly consists of 

 crinoidal limestone of the type called ' petit granit ' by the Belgian 

 geologists. Sections show that the matrix is largely dolomitized. 

 Bedding-planes covered with the brachiopods Sjjirifei' clatliratiis, 

 Orthotefes crenistria, and Chonetes hardrensis are very charac- 

 teristic of Upper Z^. Patches of crystalline calcite sometimes 

 reaching a diameter of 2 inches are frequent throughout Z,, 

 especially in the dolomitized beds, and appear to be similar in 

 character to those described by Dixon & Vaughan i from the 

 laminosa dolomites of Grower, and to be due to recrystallization of 

 calcareous mud. One band of dolomite in the low^er part of Z^ 

 contains numerous patches of quartz. The rocks at the middle of 

 Z^ are somewhat disturbed, and a thrust-plane is seen to traverse 

 them at a point immediately noi-th of Black Rock Quarry. Miss 

 M. B. Chapman gives two analyses, one of a rock containing 



^ Q. J. G. S. vol. Ixvii (1911) p. 488. 



