Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/534

450 resemblance whatever to greenstone or any other rock of the basic series. In a large quarry at the south end of Lawrence Hill thick beds of volcanic ash are seen dipping in a northerly direction, at an angle of 55°. Like all the rocks of the range, they have been greatly disturbed and fractured; but the strike of the beds is clearly across the ridge. They here consist of several alternations of coarse and fine material, the whole of which have been highly indurated and otherwise altered. These beds are covered by masses of altered pitchstones and felsites, which are well exposed on the south slope of the hill, on its steep northern face, and also on the opposite crags of Ercal Hill. Returning southwards to the narrow ravine which separates Lawrence Hill from the Wrekin proper, the ash-beds are again exposed in the precipitous face of the latter; the bedding is here massive or obliterated, although the fragmental character of the rock is still perfectly distinct. Further southwards, along the summit of the ridge, beds of the coarser ash are again seen, and may be traced at intervals for a distance of more than six hundred yards. From this point to the south-western termination of the ridge there are comparatively few exposures of rock; and these consist of compact reddish-brown felsite or altered pitchstone—the compact felspar of Murchison and others.

On the summit of the ridge, at a short distance to the north of the wood, there is a slight rounded elevation formed by a mass of altered dolerite, which appears to be intrusive; and in the quarry at the south end of Lawrence Hill there are two dykes: one, on the east side, is 12 ft. wide; the other is 14 ft. at bottom, cuts through the lower ash-beds, then bifurcates, and the two diverging branches rise through the upper beds to the surface. The rock forming both dykes is a highly altered basalt.

It is evident, therefore, from an examination of this part of the ridge, that the axis is not formed by a continuous band of greenstone, as hitherto represented, but that in reality it here consists of an extensive series of regularly stratified agglomerates and ashes alternating with amorphous masses of altered pitchstones or felsites.

The general strike of the surrounding strata is north-east; and the central ridge is flanked by masses of quartzite, which are laid down on the map as Caradoc Sandstone, altered by the supposed intrusive greenstone. Whether these rocks be altered Caradoc strata or not, they are clearly unconformable to the stratified ash-beds of the ridge; and I think there is some reason to believe that the latter belong to the older contemporaneous volcanic series so extensively developed in the Lower-Silurian district of Salop and Radnor.

The mass of trap lying to the west of Wellington and the Wrekin is a hard rock which has suffered less from denudation than the soft Triassic sandstones by which it is surrounded; it forms a low hilly tract, usually presenting rounded or flat surfaces, on which several large boulders of granite and felstone have been stranded. These erratics appear to be quite similar in character to those forming the well-known and far more numerous group just north of Wolverhampton. With the exception of some portions at the southern end