part 1" 



OF THE SHALES-WITK-' BEEF. 



80 



very distinct at the thin edge, but faint and often absent in the 

 centre. Microscopic examination shows that this furrowing is 

 superficial, and not due to fibrous structure : for each disc was 

 found to be a simple crystal flattened parallel to (001). 



These discs might be contemporaneous nodules, such as the 

 concretions of barytes dredged from recent seas ; or secondaiy 

 segregations of minute traces of barium sulphate in the shales. 

 If, however, growth had progressed freehy, it is difficult to under- 

 stand why either more complete crystal-forms or small spherules 

 did not result. On the other hand, the convex shape and the 

 absence of crj^tal form in these discs are in accordance with 

 secondary growth against vertical resistance. 1 



(C) The Veins of ' Beef.' 



The numerous veins of fibrous calcite, known as 'beef,' are the 

 most striking and interesting feature of the beds, to which they 

 have given a name. In these veins the calcite-fibres pass directly 

 from wall to wall, and they are, therefore, of the type called ' cross- 

 fibre veins.' Moreover, they possess the same general structure as 

 the cross-fibre veins of gypsum and asbestiform minerals, which 

 have been recently described. 3 A continuous plane, marked by 

 inclusions of marls, runs through all the veins, and may be called 

 the parting 3 ; and, by analogy with fibrous gypsum, the upper 

 portion of the vein may be convenientry referred to as the top, 

 and the lower as the root (fig. 4). 



The parting has been shown (in the papers just quoted) to 



represent the ori- 



Fig. 4. — Vein of 'beef,' showing structure and ginal fracture of 



cone-in-cone, Gharmouth. the matrix, from 



which the crystal- 

 lizing fibres grew 

 outwards. In the 

 case of 'beef, 'frac- 

 ture and crystal- 

 lization must have 

 been simultane- 

 ous, for the wide 

 horizontal extent 

 covered by a single 

 vein definitely 

 excludes the possibility of the existence of an open crack, even 

 for a short time. 



The veins of 'beef always occur along a bedding-plane (which 

 they only transgress when passing above or below a calcareous 



1 W. A. Eichardson, Geol. Mag. 1921, p. 120. 



2 Id. Min. Mag. vol. xix (1920) pp. 77-95 ; and S. Taber, Trans. Amer. Inst. 

 Min. Eng. vol. lvii (1918) pp. 62-98. 



:t Originally called by Dr. Taber the ' central parting ' ; but, since it rarely 

 occurs ' centrally ' in the veins, the simpler term seems preferable. 



Parting 

 Root 



