Vol. 59.] GRANITE AND GREISEN OP CLIGGA HEAD. 157 



When the quartz was deposited, a question which involves the 

 source and mode of deposition of the ores which it contains also 

 (namely, cassiterite, wolfram, mispickel, and copper-pyrites), cannot 

 be discussed here. No material advance has, it is believed, been 

 made on the well-known opinions of Daubree l and, in later years, 

 of Prof. Vogt, 2 in the matter of the formation of tin-ore, and con- 

 sequently, seeing how intimately the cassiterite is associated with 

 the other minerals formed, the actual chemical reactions which took 

 place along the ' bedding-planes.' That fluorine and boron were the 

 principal means of the metasomatism is all that we can safely say. 



To conclude then : — 



The Cligga-Head granite is the remnant of a much larger mass, 

 which has been partly denuded by marine action, and partly hidden 

 by a big north-and-south fault. 



It is possible to distinguish two divisions in the granite : the main 

 mass and the granite-tongue, throughout both of which ' bedding ' 

 is well developed. 



The granite bordering the ' bedding-planes ' has been altered to 

 greisen, which, owing to the abundance of quartz, appears in the 

 cliff-section as dark bands. 



Each greisen-band contains a quartz-vein, marking the original 

 fissure along which the metasomatism took place. These quartz-veins 

 contain blue tourmaline and the following ores: — cassiterite, wolfram, 

 mispickel, and copper-pyrites. They thus form tin -lodes which fall 

 under Prof. Vogt's ' Cornish type ' (loc. cit.) characterized by the 

 accompaniment of copper-pyrites. 



Two main reactions have taken place in the formation of the 

 greisen : the felspars affording topaz, muscovite, and secondary 

 quartz ; the biotite affording brown tourmaline, magnetite, and 

 secondary quartz. The fact that no tourmaline has been formed 

 from the felspar, owing to the presence of abundant fluorine, distin- 

 guishes this greisen from such granite-modifications as luxullianite 

 and trowlesworthite. 



The original quartz contains inclusions of minute, pale blue tour- 

 maline-prisms, sometimes orientated to the prism- and pyramid- 

 planes. They were original constituents of the granite. The 

 original quartz-grains also prove to have been enlarged by the 

 deposition of secondary quartz in optical continuity. The secondary 

 quartz has further caused the original grains to appear as if they 

 possessed a crystal-outline. 



The fluorine and boron had not so great an effect on the 

 extremity of the granite-tongue as on the main mass. This is 

 shown by the poor development of greisen and the freshness of the 

 biotite, which sometimes forms veins of porphyritic crystals. 



1 Ann. des Mines, vol. xx (1841) p. 60, & ' Etudes Synthetiques de Geologie 

 Experimentale ' vol. i (1879) p. 28. 



2 Zeitschr. f. Prakt. Geol. vol. iii (1895) p. 145. 



