Page:The South Staffordshire Coalfield - Joseph Beete Jukes - 1859.djvu/142

124 about the igneous rock would then represent that small part of the coal that was altered by having some of its gaseous constituents sucked out of it, as it were, leaving a coating of nearly pure carbon as a non-conductor to protect the remainder of the coal.

The only explanation of the occurrence of the sandstone called "rock and rig" I can offer is, that it is part of a "rock fault," a mass of sandstone contemporaneous with the coal, as described before, p. 48. Whether its connexion with the trap be accidental or otherwise it is difficult to determine. I am inclined to think, from the frequency of the occurrence of these " rock faults" in the coal in the neighbourhood of the Rowley Hills, and their absence, so far as I know, at any great distance from them, and from "rock and rig" being found around Barrow Hill, that their connexion with the trap is not accidental. It seems possible that the volcanic focus from which was subsequently protruded the molten basalt, gave some indication of its existence, even at an early period in the formation of the Coal-measures; and that from some troubled action fracturing the rocks, and either generating springs that brought up sand from below, or causing currents, perhaps of considerable local intensity, in the water 'above and around, which might sweep in sand from a distance, sandy beds might be deposited that locally interfered with the production of the Thick coal.

We have as yet no means of ascertaining the focus or centre of eruption from which the basalt of the Rowley Hills was poured out, and from which the sheets and veins of greenstone and "white rock" trap proceeded. No central pipe or funnel has yet been met with, but, except in the Netherton tunnel, no large exploration has yet been continued beneath the basalt.

The deterioration of the Thick coal by the quantity of sandstone that seems to come into it as the hills are approached is one reason for its not having been followed as yet beneath them. The fear that the coal which may exist is partly destroyed by trap rock is probably another reason.

It may still happen that at some point round the Rowley Hills good unaltered coal may be found for some distance beneath the basalt, but at present all the known evidence is the other way.

Barrow Hill.—Barrow Hill, two miles west of Dudley, is another mass of basalt in every respect similar to that of Rowley, except in extent and in its position with respect to the Thick coal. The Rowley columnar basalt seems to he at a height of about 600 feet above the Thick coal, that of Barrow Hill is apparently at a much less height. If they are each contemporaneous with the beds in which they lie, it follows that the Barrow Hill trap is older than that of the Rowley Hills. Immediately east of the hill, it appears, from the marks of old workings, that a piece of Thick coal cropped to the surface, and was worked in open work along it. This piece lay in an angle between the Corbyn's Hall fault and another, which is supposed to run south of Barrow Hill. In the workings to the west of Barrow Hill the coals are found to be "blacked" as they approach it, and beds of "green rock" (horizontal dykes) are