PART III 
EARTHY MINERALS 
CHAPTER XII 
THE MICAS, ASBESTOS, AND GEMS 
THE non-metallic minerals include many species of value 
from special physical and optical properties. 
Tur Micas 
DistriBuTioN AND Usks—The micas are anomalous in 
distribution ; they are ubiquitous in minute flakes, being 
essential constituents of many abundant rocks, such as 
granite; gneiss, mica-schist, minette, and kersantite, being 
common in micaceous sandstones, forming the bulk of some 
clays, and giving the blue colour to the Swiss lakes. Neverthe- 
less mica of industrial service is exceptionally local; it is 
found only in deep-seated pre-Paleozoic rocks; 70 per cent. 
of the world’s supplies comes from one district in India, and 
most of the rest is from two or three localities. 
The chief uses of mica are for windows and lampshades, 
where heat or vibration are too great for glass, and as an 
Insulator in electrical machinery. The most useful micas 
are the white mica, muscovite, and a brown species, phlogo- 
pite. Muscovite is usually found with pegmatite, as in 
Bihar and Nellore in India, in the Transvaal, Tanganyika 
Territory, Kenya Colony, Brazil, Quebec, the Eastern United 
States, and Russia. 
PNEUMATOLYTIC Or1GIN—The mica of Bihar is found with 
sheets, “blows ” or lenticles, and irregular masses of peg- 
matite in schist and gneissose granite. Pegmatite is some- 
times injected as a molten intrusion; but some occurrences 
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