FIRST FLOOE. 
61 
Mica (28a) is tlie name given to a group of minerals differing General 
much from each other in chemical composition and optical pro- ^^e^^als^ 
perties, but having as a common character an easy cleavage in a 
single direction, and thus affording plates remarkably thin, 
transparent, tough, and elastic. One of these minerals, musco- 
vite (28d), has been used in Eussia in place of glass for win- 
dows ; it is now in common use for lanterns and stoves, not 
being so easily cracked as glass by change of temperature : it 
is still known in commerce as talc, a term formerly applied to 
it by mineralogists, but now restricted by them to a different 
mineral. 
The group of Felspars, the most important of the rock-forming 
minerals, begins at case 28f. 
After the Felspars comes Beryl, of which the bright green 
variety. Emerald (29c), is one of the most valued of precious 
stones. It was in ancient times worked in Egypt, as is proved 
by the rough specimens found in the old workings by Sir 
Gardner Wilkinson, and presented by him to the Museum. 
Emeralds are found in the Urals ; but the locality for the finest 
stones has long been that of Muso, about seventy miles from 
Santa Ee de Bogota, in South America. Lately emeralds, though 
not of a good colour, have been discovered at Stony Point in 
North Carolina : some of the best of those found are shown in 
the case. Facetted specimens of the colourless beryl, and also 
of the bluish-green beryl, known in jewellery as Aquamarine, 
are exhibited (30a). 
In cases 30e to 32d will be found beautiful illustrations of the 
Zeolite group of minerals. 
Tourmaline (33a), when free from flaws, is, in some of its 
varieties, to be classed with the precious stones. Among these 
is the pink variety called Kubellite. Two very fine specimens 
of rubellite from Ava are shown in the case; one of them, 
remarkable for its size and shape, was brought from that country 
by Colonel Symes, to whom it had been presented by the king ; 
the other, not so large but of a deeper colour, was presented to 
the Museum by ]\Ir. C. S. J. L. Guthrie. The pink-and-green 
tourmalines from Maine, U.S.A., are among the more beautiful 
of the mineral products of the United States. Good examples 
of the blue tourmaline, Indicolite, are shown in case 33b. 
