

313 



fluor are probably in many cases secondary minerals developed in connection 

 with exhalations of boracic and fluoric acids. White mica is often developed 

 in consequence of the alteration of felspar. This mica must be carefully 

 distinguished from the white mica which occurs as an original constituent 

 of many granites. Epidote, chlorite and other minerals frequently arise 

 in connection with secondary processes. We will now refer to the granites 

 of special localities. 



West of England. Details as to the mode of occurrence of these 

 granites will be found in the Geological Report on Cornwall, Devon and 

 W. Somerset by DE LA BECHE. The petrographical characters of the 

 rocks, so far as these could be ascertained by macroscopic examination, are 

 also given in the Report and in older communications by Mr. MAJENDIE, 

 Dr. BOAZE,W Mr. CARNE and others. The microscopic characters of some 

 of the granites and of the altered rocks near the margin of the granite- 

 masses have been described by ALLPORT, (2) BoNNEY,< 3 ) PHiLLiPS, (4) RuTLEY, (5 

 and WORTH. < c > 



On the mainland of Devon and Cornwall there are five principal and 

 many smaller exposures of granite. The five principal masses may be 

 referred to as those of Dartmoor, Brown W T illy, Hensborough or St. Austel, 

 Carn Menelez and the Land's End. Notwithstanding many local variations 

 there is a general uniformity in the character of the rock from the different 

 localities. It consists essentially of quartz, orthoclase, plagioclase, dark 

 mica and white or light coloured mica. Porphyritic crystals of white 

 orthoclase are sometimes present and sometimes absent. They sometimes 

 attain very large dimensions measuring two or three inches across. The 

 prevailing colour is grey. Red tints are rare. A fine red granite occurs, 

 however, at Trowlesworthy and veins of red granite may be observed, 

 according to Mr. WORTH, near Shaugh, at Hanger Down and in the upper 

 valley of the Tavy. The grain is coarse in the central parts, but becomes 

 finer at the margins and in the apophyses. The dark mica sometimes 

 disappears and under these circumstances the rock becomes a white 

 muscovite-granite or aplite. Under the microscope zircon and apatite 

 may be detected in almost every section. Where zircon occurs as 

 inclusions in the micas it is surrounded by a coloured border; in the 

 black mica this border is frequently so dark as to be opaque, in the light- 

 coloured mica it is faint but generally distinct. In the lath-shaped 

 sections it is only seen when viewed with rays vibrating parallel to the 

 cleavages. The apatite is not surrounded by a similar border. The 

 principal constituents of the granite do not as a rule (with the exception 



(1) Trans. Roy. Soc. Corn., Vol. IV. (1832), p. 165. 



(2) On the metamorphic rocks surrounding the Land's End Mass of Granite. Q.J.G.S., 

 Vol. XXXII. (1876), p. 407. 



(3) On the microscopical structure of Luxulliaiiite. M.M. (1378), pp. 5-9. 



(4) On the rocks of the mining districts of Cornwall. Q.J.G.S., Vol. XXXI. (1875), 

 pp. 319-344. On concretionary patches and fragments of other rocks contained in Granite. 

 Q.J.G.S., Vol. XXXVI. (1880), pp. 1-21. 



(5) The Eruptive rocks of Brent Tor and its neighbourhood. Mem. Geol. Sur., 1878. 



(6) The Rocks of Plymouth. Trans. Plymouth Iiist. 1886. 



