Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/265

 from the Culm of South Wales, or the blind coal of Kilkenny; but where this coal is not covered by the greenstone, it exhibits the usual characters of common bituminous stone coal.

The beds below this coal differ in no material respect from similar beds in other collieries.

Hence it appears that the Birch-hill colliery presents the following important facts: First, the existence of a bed of greenstone interposed between the usual strata of the coal formation, but not co-extensive with them; and secondly, that the coal and bituminous shale, where they are covered by the greenstone but protected from actual contact with it by an indurated sandstone a yard in thickness, differ materially in many respects, but chiefly in being deprived of bitumen, from those parts of the same beds where they are not covered by the greenstone. The works have not indeed been sufficiently opened to demonstrate that the changes just mentioned are strictly co-extensive with the greenstone, yet I think we may infer by fair analogy that such is the case; and that the greenstone is necessarily concerned in bringing about these changes.

Professor Jameson, in his mineralogical account of Dumfriesshire, mentions beds of greenstone occurring in the independent coal formation; and I at first took for granted, that the greenstone above described was also a true bed. On further consideration, however, I am rather inclined to adopt a contrary opinion, for the following reasons:

In the first place the green rock fault being composed of precisely the same kind of greenstone as the bed is, renders it probable that the one is a continuation of the other; and this is still further confirmed by the gradual thinning out of this bed as it recedes from the fault; for in the pit B, the most distant one that has been as yet sunk through the greenstone, this bed is 12 feet thick, while in