Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/449

 south as the parallel of Bishopscastle. The general elevation of this tract above the Severn varies from 800 to above 1500 feet. It is completely intersected by two strait and simple vallies, the direction of which is about north-east and south-west, and several smaller ones parallel to the larger descend from the interior towards the north and south. The principal rock which presents itself is greywakke-slate, in beds which run north by east and south by west, rising at an angle of about 50° east by south. The colour of the rock where it has not undergone decomposition, is bluish or greyish black, probably from a mixture of carbon, for by exposure to the air it passes to a pale yellowish-grey, with a few spots of oxide of iron. Its cross fracture is dull, but its longitudinal fracture presents numerous small spangles of mica. The upper beds, or rather the superficial parts of many of the beds, are in the state of shale, that is, they are shattery, soft, and more or less decomposed. No true veins ever occur in this shale, and even veins of considerable size and regularity in the compact part of the rock, terminate almost immediately on coming in contact with the looser part.

It is in this compact greywakke-slate, and chiefly on the western side of one of the highest hills called the Stiperstones, that the principal lead mines of the county occur. Of these, that which is called the Snailbach mine, is the most important for its metallic produce, and is the only one in which witherite has hitherto been found.

The mine consists of one principal vein, with several strings proceeding from it. Its course is nearly east and west, descending at a high angle to the south: its greatest depth is about 180 yards, and there are no signs of its approaching to a termination: its general thickness is 10 or 12 feet, but in one particular part amounts to more than 30 feet. Cavities, or locks as the miners call them, are