THE ROCKS OF THE MINING DISTRICTS OF CORNWALL. 319 



20. The Rocks of the Mining Districts of Cornwall, and their 

 Relation to Metalliferous Deposits. By J. Arthur Phillips, 

 Esq., M.I.C.E., F.G.S., &c (Read March 10, 1875.) 



[Plate XVL] 



The more striking geological features of the mining districts of 

 Cornwall are so generally known as scarcely to require description. 

 The most commonly occurring rock is " killas," or clay-slate, through 

 which four large and several smaller protrusions of granite have 

 taken place*. 



The cleavage-planes of the slates almost invariably dip from the 

 intruding masses of granite, but usually at a less angle than the 

 line of eontaet of the two rocks. Fear the point of junction the 

 granite often becomes fine-grained, and not unfrequently sends off 

 veins into the adjoining slates. Masses of granite are also some- 

 times found imbedded in slate ; and fragments of slate enclosed in 

 granite are occasionally met with. At Herland, in the Crowan 

 district, at a considerable distance from any known body of granite, 

 isolated masses of this rock have been found at a depth of 110 fathomsf; 

 and somewhat similar disconnected granitic blocks are said to occur, 

 49 fathoms deep, at Huel Buller, near Redruth J. 



The granite constituting the larger areas is usually divided into 

 floors resembling beds, which form sheets in the central portions of 

 the several masses, while the edges bend beneath the surrounding 

 "sedimentary rocks, and approximate in conformation to the surface 

 of junction between the two. The granites of Cornwall and of 

 Dartmoor probably belong to the same geological age ; and there is 

 evidence that the great upheaval of the granite of the latter locality 

 occurred in Post- Carboniferous times. 



The slates in the vicinity of granite are usually of a green, brown, 

 purple, or violet hue ; but those which are situated at a distance from 

 it are often grey, bluish grey, deep blue, brownish yellow, or buff §. 

 In the western portions of the principal mining district the slates 

 are believed to be non-fossiliferous ; but in the eastern part of the 

 county various organisms occur, both in the killas and in its asso- 

 ciated beds of limestone, which indicate that these rocks belong either 

 to the Devonian or to the Lower Carboniferous period. It is, how- 

 ever, probable that some of the rocks of south-western Cornwall 

 may be of Silurian age. 



* The granite of the Germoe district, including the Godolphin and Tregoning 

 Hills, is here classed with the smaller masses. 



t W. J. Hen wood, F.R.S., Trans. Royal Geological Society of Cornwall, 

 vol. v. p. 36. 



| Ibid. p. 157- -The fragments of granite enclosed in slate may have become 

 detached from intrusive masses, and have subsequently been displaced by 

 faults. 



§ W. J. Henwood, Journal of the Royal Inst, of Cornwall, vol. iv. (1873- 

 1874) p. 10. 



Q.J.G.S. No. 123. z 



