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Micas. — Geylonese micas are of some importance from a com- 

 mercial poiat of view. They include muscovite, biotite, and 

 phlDgopite : — 



(1) Muscjuite, HjKAl3(SiO^)3 occurs but sparingly in Ceylon, 

 and not in crystals large or flawless enough to be of commercial 

 value. 



(2) Biotite, (K:H),(\l5Fe),(AlFe), (810^3 is chiefly found in 

 sm'tU crystals as a microscopic constituent of various rocks 

 belonging to the Gaarnockice Series ; but a part of vein mica also 

 belongs to biotite. 



(;i) Phlogo/jite, (K,H,Mg,F)3Mg3A.l(3iO J3 is by far the most 

 important of Jeylonese mijas. Minute pale or golden crystals are 

 almostalways common in the crystalline limestones. Larger mica 

 crystals occur in veins and bauds associated usually'' with junction 

 of granulite and crystalline Innestoue. These micas are rarely 

 colourless, being more usually brown, reddish, bottle-green, or 

 amber colourei. The largest crystals found have been two or 

 three feet in diameter. The veins are usually one or two feet 

 in width, and are composed of numerous "books " or crystals of 

 mica which are generally more or less idiomorphic, having a clear- 

 cut hexagonal outline. Mica is used for the peepholes of stoves, 

 for lamp chimneys, and very largely for electrical appliances, 

 &c. ; comminuted mica dust is of use as a non-conducting packing. 

 Mica has long been used in the East for ornamental purposes, also 

 for medicine. Fine crystals of mica from the Kandy District are 

 exhibited. 



Fig-. 5.— Tourmaline, 

 Tourmaline. — A borosilicate of alumina with magnesia iron and 

 alkalis. Black varieties associated with qnartz are not uncommon. 



