Petrology of the Shales-with-‘Beef’

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Abstract
(A) Laminated Shales Shales, which are minutely laminated at the outcrop, but appear massive when traced inwards, have been described in Part I (p. 53). The change is accompanied by progressive bleaching towards the weathered surface, and by the deposition of minute, platy crystals of selenite between the laminæ. This bleaching suggests that the development of lamination has been accompanied by a loss of certain constituents of the shales, such as carbon (or hydrocarbons), and also (as indicated by the appearance of gypsum of iron-sulphide. Thus the weathering of the shales has produced a diminution of volume, and the resulting contraction, by separating the individual laminæ, has revealed the minute internal structure of the shales. It may be noted that similar development of lamination may be seen, for example, in the Rhætic Black Shales at Beacon Hill, Newark. In bed 71 e , consisting of well-stratified blue shale, Dr. Lang has found thin biconvex discs of a mineral determined by Mr. W. Campbell Smith as barytes. These discs are irregularly distributed along bedding-surfaces, from which they are readily detached, leaving behind a perfect impression of their shape and markings. The general appearance of these barytes discs can be gathered from fig. 3. The largest specimen examined had a mean diameter of 15 mm., a maximum thickness of 2 mm., and the specific gravity = 4·58. Both the upper and lower surfaces of the discs are marked by radial furrows, very distinct at the thin edge, but faint and often absent in

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