ERUPTIVE AND METAMORPHIC ROCKS. 5/7 



and fragments of them have not unfrequently been torn off, and are now enclosed 

 in the granite. There can be no question, therefore, that the granite here presents 

 all the characters of an intrusive igneous rock.^ 



At VVicca Pool, Zennor, veins of granite penetrate the mica-slates, and fragments 

 of the latter, that have been detached from the general mass, have become enclosed 

 in the granite.''^ In Devonshire the slaty rocks are altered in places in proximity 

 to the granite. (See p. 199, and Fig. 30, p. 194.) 



The term " Elvan " is used in Cornwall and Devon for varieties of quartz- 

 felsite or quartz-porphyry, generally of a whitey-brown colour, that occur in veins 

 or dykes. These dykes vary in width from a few feet to several fathoms, and 

 traverse both granite and slates ; they often coincide in direction with lodes. ^ At 

 the Caradon Mines hand-specimens can be obtained showing the junction of 

 elvan and granite. (See p. 566.) 



Elvans vary considerably in texture ; sometimes they are of a fissile nature. 

 These rocks have been obtained from the Land's End, from Marazion near 

 Penzance, near Helston, Penryn, Truro, Pentuan (Pentuan Stone), Lanivet, St. 

 Neot's, St. Dennis, Camelford, Oakhampton, Rowborough (Rovvborough stone) 

 between Plymouth and Tavistock, etc. Elvan is a very durable stone, and is well 

 adapted for building- purposes and for road-metal. 



In Central and Eastern Cornwall Mr. J. A. Phillips has found lava-flows so inter- 

 bedded with the slates and schists as to indicate that they were contemporaneous. 

 The lavas sometimes assume a distinctly schistose structure. Most of the so-called 

 "greenstones" appear to be altered dolerites. On St. Cleer Downs the "horn- 

 blende slates" graduate imperceptibly from crystalline dolerite on the one hand, 

 into clay-slates on the other ; and instances are by no means wanting where a rock 

 is massive and crystalline near the centre of its outcrop, while externally it is 

 schistose and without visible crystals. Mr. Phillips adds that this schistosity in 

 igneous rocks is often exceedingly puzzling, as it is sometimes impossible to 

 determine where foliated traps cease and where metamorphosed slates begin. 



Dolerites are exposed west of St. Austell, and quarried for road-metal at Hill 

 Head, near Camelford, and Trelill, St. Kevv. In places they are called " Dun- 

 stones." Their age is generally greater than that of the granite. Some of the 

 so-called "greenstones" of western Cornwall are gabbros or dolerites and horn- 

 blendic or chloritic slates :* and these slates or schists are possibly of Archjean 

 age. (See p. 46.) Mr. Rutley has described some augite-andesites from the 

 neighbourhood of St. Minver, near Padstow.^ 



Serpentine, which is a hydrous silicate of magnesia, etc., is so called from the 

 fancied resemblance of the rock to a serpent's skin. It occurs both as a mineral 

 and as a rock. It is generally regarded as the most beautiful of the ornamental 

 stones of this country, and even in its natural position, in the Lizard district, and 

 especially in Kynance Cove, the varied hues of the rock appear in pleasing contrast 

 with the white sands of the beach. Red and green varieties of serpentine are 

 found, and the rock contains also diallage, veins of steatite, asbestus, etc. 

 (See p. 56S.) It may be seen also at Coverack and Kennack Coves, at Mullion, 

 Cadgwith, St. Keverne, Nare Point, Poltreath, Porthalia, and Polkerris. 

 In De la Beche's opinion the rock was a product of metamorphism,*" and 

 this view has been supported by Mr. J. H. Collins,^ by Prof. W. King, and 

 Dr. T. H. Rovvney : * but Prof. Bonney is of opinion that it was originally an 

 igneous rock, intrusive among the hornblende schists, and broken through 

 in places by granitic dykes. 



^ S. Allport, Q. J. xxxii. 407 ; see also J. A. Phillips, Q.J. xxxi. 338. 



2 J. A. Phillips, Q. J. xxxvi. 9. 



^ J. A. Phillips, Q. J. xxxi 334. 



* J. A. Phillips, Q. J. xxxi. 328, xxxii. 155, xxxiv. 471 ; S. Allport, Q. J. 

 xxxii. 407 ; L. Horner, Q. J. x. 359. 



5 Q J. xlii. 392. 



^ Report Geol. Cornwall, etc., p. 30. See also Teall, Brit. Petrogr. PI. xv. 

 and p. 104. 



' G. Mag. 1S85, p. 298, 1886, p. 359 ; Q. J. xl. 458. 



8 Phil. Mag. (5), i. 280. 



37 



