576 



ERUPTIVE AND METAMORPHIC ROCKS. 



the normal granite of the country.^ This rock was used for the Duke of 

 WelHngton's sarcophagus in St. Paul's Cathedral. 



Granite has been raised on Dartmoor at the extensive quarries of Foggen Tor, 

 near Prince's Town, also at Fremator, near Tavistock, at Hey (High) Tor, etc. 

 Dartmoor granite was employed in constructing the Thames embankment. 



The decomposition of granite has at various periods given rise to accumulations 

 of clay, the purer kinds of which are known as Ivaolin or China Clay. This clay 

 results from the decomposition of the felspar and mica in the granite, but the term 

 China Stone is sometimes applied to the material when it retains much of the 

 quartz and is in a less advanced state of disintegration. On the other hand, the 

 term Arkose is applied to a felspathic grit, due to the disintegration of Ciranite and 

 the subsequent cementing of the materials in sitil, or nearly so: quartz and felspar 

 predominate in this material, and mica is sometimes present. 



Extensive deposits of China Clay (Cornish or Porcelain Clay) occur at St. 

 Austell, Hingston Down near Calstock, and at Morley and Lee Moor near 

 Plympton, in Devonshire. The clay is employed in the manufacture of porcelain 

 and porcelain fire-bricks ; it is used in paper manufactories and in bleaching 

 establishments.- (See also p. 444 ) 



Granite has been quarried at Lundy Island ; the Gannet Stone (Ganiston) is a 

 variety worked on the northern part of it. The island, which is about 32 miles 

 long, and from 500 to 600 feet above the sea-level, is formed chiefly of Granite, 

 excepting the south-east corner, which is Devonian clay-slate. (See p. 129.) The 

 granite appears to be bedded, occurring in great piles or "cheeses," features due 

 to the joints and weathering. Dykes of greenstone also occur in the island.' 



From the Channel Islands large quantities of granite are exported, chiefly for 

 use as London road metal. It is obtained from the quarries of Mont Mado and 

 La Perruque in Jersey, and from Guernsey and the little island of Herm. Pink 

 and grey granite is imported from St. Sampson's Bay in Guernsey, and known as 

 Guernsey or St. Sampson's stone. At Jersey china clay has been obtained, and 

 bricks are made. There is much gneiss in the southern part of Guernsey, while 

 the northern part is largely made up of Diorite and .Syenite, with Granite and 

 Hornblende Gablno ("iJird's Eye"^). Gneiss occurs in Sark, and also Horn- 

 blendic Schist, which may be of Archaean age. (See p. 46.) Syenite occurs also 

 in Alderney and Sark. 



The Scilly Isles comprise about one hundred and forty-five islets or rocks, 

 of which St. Mary, the largest, is about 3 miles long by 2| miles broad, attains 

 a height of about 204 feet above the sea-level, and occupies an area of about 1640 

 acres It is almost exclusively granitic, but shows traces of clay-slate. The Gulf 

 or Wolf rock, situated between the Land's End and the Scilly Islands, is formed 

 of phonolite. The granite is of coarse appearance, and is therefore not largely 

 quarried.* 



Mr. Aliport has observed that on approaching either of the granite masses of 

 Cornwall, the clay-slatts become more and more indurated, and they are traversed 

 by numerous quartz-veins : mica and schorl make their appearance in the slates, 

 and at the junction ot the two rocks, their slaty character has in many cases been 

 obliterated. At Mousehole, St. Michael's Mount, and Cape Cornwall, the mass 

 of granite cuts sharjily through the slates, and has thrown out numerous veins, 

 both large and small, which have penetrated them in various directions. In all 

 such cases the slates have evidently been greatly altered along the line of junction ; 



^ Mineralogical Mag. i. 215 ; Address to Geol. Soc, 1886, p. 59 ; see also 

 Pisani, Comptes Rendus, 1864. 



- See G. Mag. 1867, p. 241 ; P. Geol. Assoc, viii. 479; R. Hunt and F. W. 

 Rudler, Guide Mus. I'ract. Geol. ed. 4, p. I41. 



3 T. M. Hall, Trans. Devon Assoc. 1871. 



« Rev. E. Hill, Q. J. xl. 404; J A. Birds, G. Mag. 187S, p. 79; MacCulloch, 

 T. G. S. i. 12 ; A. de Lapfiarent, Bull. Soc. (leol. France (3), xii. 284 ; Prof. G. 

 D. Liveing, Proc. Cam!)r. Phil. Soc. iv. ; C. Noury, Geologie de Jersey, 1886. 



5 Kev. F. F. Staliiam, Geologist, ii 12 ; De la Beche, Mem. Geol. Survey, 

 i. 233, and Report, pp. 161, 494 ; Joseph Carne, Trans. R. G. S. Cornwall, 

 vii. 140. 



