Page:The South Staffordshire Coalfield - Joseph Beete Jukes - 1859.djvu/148

130 masses of this white trap also are found in the pits near Birch Hills Hall, north of Walsall. At Union colliery, north of that, the Bottom coal is cut entirely out by "green rock;" and at Goscott, still farther north, there are six yards of "green rock" resting directly on the Bottom coal. White rock trap just shows itself in the cutting of the Cannock railway south of the bridge in the lane leading from Landywood to the turnpike road. In the rest of the field I have found no trace of the rock having been met with, and it certainly has not been seen in any of the Brereton workings at its northern apex.

Time and Mode of Formation of the Trap Rocks.—There is nothing in the mineralogical constitution of, these igneous rocks that will give us any assistance in determining the geological period during which they were formed. I am not aware that the basalts of the Rowley Hills differ in any essential particular from those of the county Antrim (the Giant's Causeway, &c.), from those poured forth by existing volcanoes, or from basalt of any other period. The greenstone does not appear to differ -essentially from the rock so called, which was formed during any period, from the Lower Silurian down to the most recent.

There appears, however, to be this difference in the circumstances under which basalt and greenstone were at any time formed: that while basalt is found in the lower part of the lava streams of existing and extinct volcanoes, and is therefore capable of being produced by the cooling of a molten mass on the surface of the earth, greenstone is not known to have been ever found to be so circumstanced, but always in such situations as either prove it to be an intrusive and comparatively deep-seated, or, at all events, not a superficially formed rock, or at least render it probable or possible that it was so formed.

Whether the difference between basalt and greenstone can be accounted for solely by the difference of the circumstances under which they cooled and consolidated from a molten mass into a solid rock, is a question I do not pretend to decide. My own belief is, that those circumstances exercised a preponderating 4nfluence on the distinction between the two rocks, and this belief is supported rather than opposed by the facts to be observed in the South Staffordshire coal-field.

If the ashy-looking beds associated with the Rowley basalt be really of the nature of "ash" or "tuff," then it follows, as a consequence, that the Rowley basalt is part of an actual lava stream poured out at the surface, either into the air or into the water. The time of this ejection was apparently after the formation of 600 or 700 feet of Coal-measures over the Thick coal, probably after the deposition of the red coal-measure clays, and about the commencement of that of the Halesowen sandstone group.