Page:The South Staffordshire Coalfield - Joseph Beete Jukes - 1859.djvu/185

Rh is known either of the constitution or the "lic" of the Coal-measures, but they are believed to dip gently to the southward until they pass beneath the red rock found above the Lappal tunnel. These red rocks may be seen occasionally thereabouts to dip south at an angle of 5° to the south.

From the neighbourhood of the Lappal tunnel, however, it will be seen, by reference to the map, that a curious little narrow strip of Coal-measures runs out two miles to the east, as far as the Stone House near Harborne. The existence of Coal-measures at the surface along this strip is shown partly by the rubbish got out of the shafts of the tunnel near Wilderness Farm, but principally by a pit sunk by Mr. Flavel, between the Stone House and Bog Meadow Coppice. From this pit, which was 80 yards deep, nothing but Coal-measure shales and "binds," with small ironstone nodules, was extracted. The measures dipped north at about 10°, and they may possibly have dipped regularly under the Permian and New red sandstone rocks which stretch from Harborne to the Quinton, though it is much more likely that the boundary between them is a fault. "On the south side of the ridge, at all events, the boundary must certainly be a fault, as near the Stone House brick red sandstone may be seen dipping south at 10°; flattening to 5° at the quarry beyond Weoley Castle.

The northern portion of the coal-field.—The northern portion of the coal-field is the part between the Great Bentley fault and Brereton. None of the faults we have hitherto been examining, with the exception perhaps of the Russell's Hall fault, exercise any marked influence on any large portion of the country. They merely traverse the beds for a certain small distance, and alter their levels over a certain limited space, the amount of the alteration soon diminishing to nothing. If either of these faults did not exist, there would rarely be any material change in the nature or character of the rocks of the locality.

With the Great Bentley fault, however, the case is different.

We have seen that between Wolverhampton and Walsall the Coal-measures crop gradually, but steadily, to the north, so that, first the Thick coal, then the Heathen, then the New mine, rise to the surface of the ground and end towards the north. The Fire-clay and Bottom coals are but a few yards deep about Bentley Lodge and Deepmore Coppice; and it is clear that if this gradual rise of the measures to the north had continued uninterruptedly for a comparatively short distance further, the very base of the Coal-measures would have come up to the surface of the ground, and the Silurian shales and limestones would then have cropped out beyond, spreading north perhaps for many miles. Cannock Chase would in that case have been a Silurian instead of a Coal-measure country.

The great dislocation of the Bentley fault, however, throwing down to the north to the amount of 120 yards (360 feet), brings in the Coal-measures even up to the base of the Thick coal, and that formation then spreads to the north, forming the surface of the ground, till it is concealed under the New red sandstone.

About a quarter of a mile north of the Great Bentley fault another runs parallel to it, throwing down to the south 25 yards, and to that extent forming a trough, or neutralizing the effect of the northern downthrow; still, however, leaving nearly 100 yards of downthrow to benefit the country on the northern side of the trough. (See Fig. 25.)