In the first six of these sections there is a remarkable agreement in the total thickness of Coal-measures below the Bottom coal, supposing that in each case the "blue binds" or "blue rock," from its colour and its preserving a uniformity of structure through so much greater thickness than the Coal-measures usually do, be the Silurian shale. At Parkfield (No. 7), the beds below the Bottom coal are thicker than elsewhere, in consequence of the extra thickness in those between the Blue flats and the Silurian shale. At Chillington (No. 8), we have no means of ascertaining this point, or of saying anything more than that they were in the Silurian shale when they got down 240 feet below the Blue flats.
At Bentley, the Coal-measure beds below the Bottom coal have thickened to 146 feet, with 30 feet below the Blue flats group, while at Ryecroft there is only 6 feet of Coal-measures between that group and the top of the Silurian shale.
In all cases, the lowest beds of Coal-measures, where ascertainable, were found to be sandstone, generally containing pebbles.
Thickening in the lower beds of the Coal-measures as they range from south to north—In page 20 is given a table of the beds, with the minimum and maximum thickness of each, as described in the foregoing pages. If, looking at those below the Thick coal, the minima were added together, we should only have a thickness of 98 feet for the whole of the beds between the bottom of the Thick coal and the top of the Blue flats. If the maxima were added, on the contrary, we should have a total thickness of 539 feet. The mean of these two numbers is 318. Now it is a remarkable instance of the way in which the frequent variations in the thickness and character of the beds are equalized among themselves, and a certain mean thickness kept, that the mean of 13 pit sections, distributed pretty equally over various parts of the district, gives 322 feet as the actual mean thickness of the beds between the base of the Thick coal and the top of the Blue flats. If, moreover, we arrange these 13 sections in the order of their thicknesses, we shall find that that will very nearly be the order of their latitude, the thinnest section being the most southerly and the thickest the most northerly; the others being nearly regularly arranged between them. I add at the beginning a section still further south, at Foxyards, where it was doubtful whether the ironstone were the Blue flats or not; the probability of its being so having become much greater from the way in which it harmonizes with the other sections.
| Thickness between the Thick Coal and Blue Flats in Feet. | ||
| 1. | Foxyards | 240 |
| 2. | Bradley | 270 |
| 3. | Deepfields | 274 |
| 4. | Crabtree piece, near Bilston | 290 |
| 5. | Stowheath. | 300 |
| 6. | Friezland | 302 |
| 7. | Stowheath | 306 |
| 8. | Chillington Colliery | 312 |
| 9. | Rough Hills | 329 |
| 10. | Priestfield | 333 |
| 11. | Ettingshall Lodge | 336 |
| 12. | Bentley Estate | 365 |
| 13. | Bentley, another shaft | 378 |
| 14. | Bloxwich | 395 |
