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. 



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