364 ME. H. H. ARXOLD-BEMROSE ON [Allg. I904, 



26. On some Quartzite-Dykes in Mountain-Limestone near 

 Snelstox (Derbyshire). By Henry Howe Arnold-Bemrose, 

 Esq., M.A., F.G.S. (Read May 11th, 1904.) 



[Plates XXX & XXXI.] 



I. Description of the Quarry axd the Dykes. 



About 34 miles south-west by south of Ashbourne, near Snelston 

 Common, is an inlier of Mountain-Limestone surrounded by Keuper 

 Marl. According to the Geological-Survey Map, the outcrop of 

 limestone is roughly elliptical in shape, the major axis extending 

 for a distance of about half a mile in a north-north-easterly 

 direction, and the minor axis in a west-north-westerly direction 

 for about one eighth of a mile. Cockshead Lane, the road from 

 Xorbury to Cubley Common, passes over the inlier, which only forms 

 a slight feature in the landscape, at a height of about 600 feet above 

 Ordnance-datum. On a clear day, some of the churches and chimneys 

 of Derby, distaut about 11 miles, can be seen from the top of the 



quarry. 



The limestone has been quarried on both the north and south 

 sides of Cockshead Lane for about 40 feet below the ground-level, 

 so that the lane passes over a high wall of limestone-beds, which 

 have been left intact between the two quarry-floors. The northern 

 quarry is now disused. It contains lead- and copper-ores, which 

 were worked about 30 years ago. It is outside the area of the main 

 mass of Mountain-Limestone to which the curious lead-mining laws 

 apply, and I was informed that the present owner does not allow 

 the lead-ore to be worked. 



The quarry south of Cockshead Lane is being worked for road- 

 metal. The inlier is in the form of a dome or pericline, with its 

 greatest extension in a north-north-easterly direction. Only a 

 brief examination is necessary to show that the limestone varies 

 considerably in character in different parts of the quarry. It is 

 generally a massive limestone, with a few chert-nodules in the upper 

 part ; at the south-western end of the quarry the grey limestones 

 are seen dipping south-westward at an angle of 30°. They are 

 probably the highest beds in the quarry. The central portion of 

 the dome has been removed down to the present floor, so that it is 

 uncertain whether the thin limestones extended over the whole area 

 at the time when the quarry was first opened, or whether they had 

 been removed by denudation. 



Some parts of the rock-face show a slickensided surface, due to 

 differential movement on opposite sides of the nearly-vertical 

 joints. The rock in many places has a broken appearance, and 

 contains small hollow spaces or caverns ; large portions of the 

 limestone have been partly or completely dolomitized, and are of 



