92 Mineral gallery. 



much from each other in chemical composition and optical pro* 

 perties, but having as a common character an easy splitting, or 

 cleavage, in a single direction, and thus affording plates re- 

 markably thin, transparent, tough, and elastic. One of these 

 minerals, Muscovite (28d), has been used in Eussia in place of 

 glass for windows, and is now in common use for lanterns and 

 stoves, not being so easUy cracked as glass by changes of 

 temperature ; is often known in commerce as talc, a term 

 restricted by mineralogists 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 specimens found in the 

 old workings by Sir Gardner Willcinson, and presented by 

 him to the Museum. Emeralds occur in the Urals ; but the 

 locality for the finest stones has long been Muso, about 

 seventy miles from Santa Fe de Bogota, in South America. 

 Emeralds, though not of a good colour, also occur at Stony 

 Point, North Carolina. 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 examples 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 Eubellite. Two fine specimens of 

 rubellite from Burma are shown in the case ; one, 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 Mr. 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. 

 Examples of the blue tourmaline, Indicolite, are shown in 

 case 33b. 



A rich blue mineral, the Lapis-Lazuli of jewellery (34b), 

 brought from Persia, China, Siberia, Bokhara, and Chili, 

 is a mixture of various species. When powdered, lapis- 



