more absurd would it be to attempt to find it by sinking in the Lias, as was formerly tried on Needwood forest.[1] It would even be a very imprudent speculation to attempt to sink for coal within any part of the New red sandstone district, unless it were first clearly ascertained that no great thickness of Permian rocks was likely to occur between the bottom of the New red sandstone and the top of the Coal-measures.
Supposing any one to be desirous of sinking for coal, either in the district between the South Staffordshire and Shropshire, or between the South Staffordshire and Warwickshire coal-fields, and assuming that Coal-measures stretch without interruption beneath, an assumption which the exposed area of the district described would not entirely warrant, he will have to calculate—
1st. The probable thickness of the beds of the New red sandstone he will have to pass through.—This will, in many places, be several hundred feet, let us say 500.
2nd. The possible or probable thickness of the Permian formation.—We have seen reason to suspect that this is in some places, perhaps in many, at least 500 yards, or 1,500 feet in thickness. He might be lucky enough to hit upon a spot where there was none of this; but there would be a great chance against such 4 piece of good fortune, and he would only act wisely to set this rock down as 1,500 feet thick.
3rd. The probable thickness of the upper and unproductive Coal-measures.— We have seen that in some part of the South Staffordshire coal-field there are 900 or 1,000 feet of Coal-measures above any of the workable beds of coal or ironstone. Our supposed speculator then would, in any place, have a great chance of coming first upon these upper measures, and would do well to calculate on the possible occurrence of 1,000 feet of them before he reached the more profitable beds.
We thus get altogether a total of 3,000 feet, or 1,000 yards, for the probable depth of good workable Coal-measures over the greater part of the space between the South Staffordshire coalfield and those of Shropshire on the one hand, or that of Warwickshire on the other, under the supposition above mentioned.
He must also take into account that after sinking for perhaps 1,000 or 2,000 feet (say 600 or 700 yards) through red rocks, he might find that he had come down upon a place where the Coal-measures are altogether absent, and might find himself penetrating Silurian shale or some other formation that lies below the coal.
- ↑ I may, perhaps, be pardoned for saying here, that even within my own recollection, since the year 1830 for instance, more money has been foolishly expended in the abortive search for coal alone, in places where a geologist would have at once declared against the possibility of finding it, than would have paid the entire cost of the Geological Survey from its commencement to the present time, or its future cost for many years.
Any search for coal through the lias or through the red mar! could only be advisable in localities where those formations could be shown by geologists to have, probably or certainly, overlapped the Bunter and Permian beds, and to rest directly on the Coal-measures.
