a smooth floor for the deposition of the chief mass of the Coal— measure beds. (See p. 80.)
The Coal-measures, then, are distinctly unconformable to the Silurian rocks, but never so much so as that their respective angles of inclination are strikingly different at any one locality.
The practical importance of these apparently quite theoretical discussions will admit of a very apt illustration in this district. If the reader will look in the map of the district, at Walsall and its neighbourhood, he will see the Silurian limestones peeping out from under the Coal-measures here and there, and again becoming concealed by them, the Coal-measure boundary not at all strictly following the line of the limestones. Let us suppose the Coal-measure boundary to represent the outcrop of the Blue flats ironstone, which it really does very nearly, and let a sinking be made, say at the Butts, near Rushall, in which sinking, after passing through the Blue flats, the limestone was found at a certain distance, say 30 yards, below it; a person not understanding the fact of the unconformability of the two formations, might, after getting the Blue flats further east near Caudy-fields, sink down again for the limestone, and feel absolutely certain of reaching it at some depth not varying greatly from 30 yards. Instead of finding it, the pit would be sunk beyond the outcrop of the limestone altogether, as in the following diagram.
Fig. 17.
A. The first shaft.
1. Coal-measures.
2 Blue flats.
3. Lowest Coal-measures.
B. The second ditto.
4. Silurian shale.
5. Silurian limestone.
Now this very case absolutely occurred, although not exactly in the method above stated, and was described to me by Mr. Roberts, the mine agent at that very locality. It had always been a rather puzzling and incomprehensible occurrence to him, till I explained the way in which it had taken place.[1]
Having established the fact of the general unconformability between the Coal-measures and Silurian rocks, let us now examine the relations between the Coal-measures and the rocks above them, namely, the Permian and the New red sandstone.
1. As to the Permian.—We have already seen that in the southern part of the district we had nearly or quite a thousand feet of Coal-measures above the Thick coal, without including any Permian, or any other rock than the true Coal-measures. Now, in the sinkings at West Bromwich that took place a few years ago, several shafts passed down through a considerable thickness of Permian sandstone, and through a part of the Coal-measures
- ↑ This is one of the numerous minor difficulties in the way of intelligent practical men that I heard of while surveying this coal-field, that would have been no difficulties, had the knowledge of a little "practical geology" been attainable by them at an early period of life.
