part 1] 



OF THE SHALES-^'ITlI-' BEEF. 



91 



(D) The Cone-in-Cone Structure in the ' Beef/ 



Among cross-fibre veins cone-in-cone structure is confined to 

 those in which the mineral is calcite. Moreover, wherever cone-in- 

 cone is found, the material is always fibrous or acicular calcite. 

 The structure is never absent in the ' beef '-veins under discussion. 

 The thinnest veins show the simpler forms, and, as the vein increases 

 in thickness, cone-in-cone structure increases in complexity. 

 Further, a comparison of museum specimens reveals the fact that 

 the big specimens from the Coal Measures and elsewhere are 

 exceedingly complex. So frequently is this relation observed that 

 complexity in the development of cone-in-cone is 

 probably dependent largely on the thickness of the 

 deposit. 



Examination of hand-specimens of ' beef ' show that the cones 

 themselves pay no regard to the parting (fig. 4, p. 89), but pass 

 without break or deviation from the upper to the lower wall of the 

 vein. There is, moreover, neither faulting on a microscopic scale, 

 nor any kind of displacement of the parting by the cones. On 

 the contrary, the fibres themselves all terminate at the parting, 

 and are never continued across it. 



Fig. 5. — Two types of simple cone-structure in 'beef? 



The apical angle of the cones varies, but lies generally between 

 50° and 60°. In the simpler types the cone's consist of bundles 

 of fibres, so arranged that they are nearly parallel one to the other 

 and normal to the wall of the vein. In this case the cone-like 

 character is apparently produced by fibres of different length 

 terminating at the outer surfaces of the cones (fig. 5, a). 



In the larger examples the fibres are notably inclined, tending 

 to arrange themselves parallel to the sides of the cones, in the 

 middle of which the fibres meet, showing a characteristic V-pattern 

 on fractured surfaces (fig. 5, b). In still larger examples this 

 mutual inclination of the calcite-fibres is greater, and more intense 

 crowding results in the appearance of smaller conic surfaces within 

 the initial cone, leading to that complex structure aptly described 

 as cone -in -cone. 



It should be noted that cone-in-cone is not especially associated 

 with, or developed in, zones where the strata have been disturbed. 

 But the effects of crushing are not uncommon in veins from such 



