40 
Records of the Geological Survey of India. 
[vol. VII. 
innumerable dykes and veins, as well as in larger masses. The rock is generally a very coarse 
quaternary compound, composed of quartz, felspar, mica, and tourmaline, united in very 
varying proportions. 
Tke quartz is wliite and translucent to semi-transparent, and never, so far as I have 
observed, presents any approach to crystallization. The felspar is sometimes orthoclase, 
sometimes albite, the latter having an opaque white color: the orthoclase is also generally 
white, but of a less pure tint; occasionally it is more or less reddish. The mica (muscovite) 
generally has a smoke brown color in plates of moderate thickness, although colorless in thin 
laminse. When perfectly unaltered and free from internal foreign matter, it is highly trans¬ 
parent, but where decomposed, it loses some of its diaphaniety and acquires a more silvery 
lustre. The plates occur of every size, up to 18 inches diameter or more, hut such very 
large ones are much less common as those of a few inches across. Occasionally, two minor 
cleavages are apparent (oo P and oo Pco), parallel to the latter of which more especially, the 
mineral divides into narrow ribbons, and fibres like asbestos, or, wkero both are present, it is 
divided by them into equilateral triangles. Greenish-gray beautifully plumose mica is also 
not uncommon, weathering out in small irregular masses above the general surface of the 
rock. Dark-brown and olive-green biotito sometimes occurs, but even where most plentiful, it 
is quite subordinate to tho muscovite, which it never altogether replaces. Occasionally the 
plates are some inches across, and include smaller interlaminated ones of muscovite. 
Tourmaline is rarely entirely absent from the pegmatite, and most usually forms an 
important ingredient in it. The crystals often attain a largo size; those of two and throe 
inches across are common, and in some dykes they are met with over six inches diameter. 
Owing to the great brittleness of this mineral, in comparison to the felspar and quartz in 
which it is imbedded, crystals approaching perfection are rarely obtainable; the few I did 
secure were of the common form oo P2. oo R, — JR. Sometimes the prisms lie parallel 
to each other and perpendicular to the walls of the vein, hut this is far from being universal, 
or even common, and it seems, as might be expected, to be more usually observable in dykes 
of a few feet in thickness. The tourmaline is jet black with brilliant lustre, and the large 
lumps often met with in the mica mines are superficially not unlike anthracite ; some of the 
miners who have seen the Karharbari coal-field take them to be coal, hut few of them have 
any idea of what real coal is like. Small crystals of tourmaline are sometimes found imbedded 
in plates of mica, with their principal axis parallel to the cleavage of the latter; crystals are 
again observable penetrating others of the same species. It appears that the tourmaline was 
generally the first to crystallize, the mica next, afterwards the felspar, and the quartz to have 
resulted last. 
The relative proportions of the different minerals vary greatly; generally all four are pre¬ 
sent, but in some places the rock consists chiefly of felspar and mica with little quartz, in 
others it is made up entirely of quartz and mica, and the latter again diminishes in amount 
until the rock passes into micaceous or into pure vein quartz. Graphic granite, composed of 
felspar- with a little quartz, is another variety occasionally met with. Sometimes the tourma¬ 
line is absent, in other cases it is one of the most prominent constituents of the rock. 
The pegmatite, as a whole, is very largely crystallized, hut one of its most marked 
characteristics is its unevenness of texture. In one place it may be comparatively fine, hut 
here within a few feet a great mass of pure felspar, with cleavage faces a foot long, occurs, and 
there another of translucent quartz, or perhaps these contain plates of mica over a foot across. 
It is worthy of note that the coarsest pegmatite often occurs in dykes of only a few yards in 
breadth, not in the large granitic masses, and it is in the dykes consequently that nearly all 
of the mica mines have been sunk. 
