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GEOLOGY them before bringing to a close the account of the igneous rocks of the district. One of the most interesting of these sills is in Dam Dale, a short distance south of Peak Forest village. The igneous rock is exposed in a small valley, and dips regularly beneath the limestone. The thickness is unknown, as the base is not visible, and the igneous rock is not seen to cut across the limestone beds in this place. The limestones immediately above it are marmorized for a distance of about 5 feet, whilst a few feet higher in the series they contain chert nodules and are partly dolomitized and show no signs of contact metamorphism. The marmorization may be traced for a horizontal distance of 800 feet on the north-east of the outcrop, and for about half that distance on the north-west on the opposite side of the valley. The rock is a coarse grained ophitic dolerite which becomes fine grained near its upper margin. About a mile south-east of Peak Forest sill a coarse ophitic dolerite covers nearly ninety acres of the surface. In the north-west it is bedded with the limestone, but on the south-east it cuts across the beds. The limestones immediately above it on the north-west are marmorized. It varies in thickness, and in some places evidently forms only a thin coating over the limestone below it, which is seen in several small quarries and swallow holes. At Black Hillock a shaft is said to have been sunk 100 fathoms into it without reaching the bottom, whilst a short distance to the north-east it varied in thickness from 16 fathoms to 2 fathoms. The Black Hillock shaft was probably sunk down the pipe up which the igneous mass found its way. In Tideswell Dale there is another interesting sill. The ground is somewhat complicated. The sill occurs in an inlier of Mountain Lime- stone which has been brought up by two faults. Intercalated with the limestones is a bed of red clay, which varies in thickness and is in places absent. This appears to have been followed by several lava flows. At a later period the intrusive rock made its way into the lava and spread along planes of weakness. It occupies different horizons in the lava, sometimes resting on the limestone, at others on the clay, and at others on the vesicular lava. Below the sill the clay has been baked to a depth of 9 feet, and the limestone has been altered to a hard saccharoidal marble to a depth of 10 or 12 feet. Where some feet of the vesicular or slaggy lava separates the intrusive rock from the clay or the limestone underneath it, no alteration has been produced in the latter rocks. The sill is about 70 feet thick. It is well exposed in an old marble quarry. The central portions are a coarse grained ophitic dolerite, but the upper and lower margins pass into a fine grained dolerite. Several other outcrops of ophitic dolerite in the county are probably intrusive bosses or sills. One of these near Ible and others at Bonsall and Buxton transgress the beds of limestone in their neighbour- hood. 19