30 HOLLAND : MICA DEPOSITS OF INDIA. 



III.-GEOLOGICAL OCCURRENCE. 



The commonest occurrence of muscovite is as a constituent of 

 granite, w hich may be so fine in grain that the individual minerals can 

 be recognised only with a microscope, or so excessively coarse that 

 the crystals may measure several feet across. The latter form, on 

 account of the size of the crystals, is known as giant-granite or granite- 

 pegmatite, and it more usually occurs in dyke-like messes, lenses or 

 veins, not generally in the form of large irregular bosses like the com- 

 moner, massive, fine-grained granite. 



The term pegmatite was originally proposed in 1822 by the French 

 Abbe Haiiy 3 for the peculiar intergrowth of quartz and felspar now 

 known as graphic granite; but in 1849 the term was extended in 

 meaning by Delesse 2 to cover coarse-grained veins containing silvery 

 mica and often tourmaline, as well as quartz and felspar. The name 

 thus became used to indicate the large size of the crystals irrespective 

 of any peculiarity of structure, and lately its meaning has been used in 

 a still more general sense to cover the coarsely crystallized varieties of 

 other forms of plutonic igneous rocks like coarse-grained syenite, 

 diorite, gabbro, etc., the variations in composition being indicated by 

 the use of compound names, as granite-pegmatite, syenite-pegmatite, 

 diorite-pegmatite, etc. 8 



The pegmatites which contain muscovite in large crystals are 

 exclusively acid (siliceous) in composition, having in general the 

 mineral composition of granite. The only pegmatites we are concerned 

 with in connection with mica belong, therefore, to the class of granite- 



1 Traite de Mineralogie, 2nd Ed., Vol. IV, p. 436. 



2 Delesse, " Sur la pegmatite avec tourmaline de Saint Etienne (Vosges)." 

 Ann, des Mines, 4th ser., XVI, 97. 



3 Cf. W. C. Brogger, " Die Syenitpegmatitgange der sudnorwegischen 

 Augit und Nephelinsyenite", Zeitschfiir Kryst., XVI, 1890. G. H. Williams, 

 55 tieneral relations of the granitic rocks in the Middle Atlantic Piedmont Plateau," 

 15th Ann. Rep. U. S. Geol. Surv., 675 (1894). H. Rosenbusch, " MiUroskopische 

 Phys. der mass. Gest., 1896, 492-497 and Gesteinslehre, 1S98, 220. 



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