Gibbs on Tourmaline, ire, 349 



The analysis of Professor Stromeyer, of Gottingea, gives, 



Silex 70.68 



Alumine 19.80 



Soda 9.05 



Iron, Mag. and Lime . 38 



99.91 



The chief difference between this and the adularia is, that 

 one contains fourteen potash and the other nine soda. Be- 

 tween this and the saussurite, or tenacious feldspar, the one 

 contains eleven of lime, and the other only a trace. 



The silicious feldspar, which I suspect to be the btKis of the 

 granite, crystallizes in thin rhomboidal tables. They are very 

 frangible, and have one clivage perpendicular to the faces of 

 the tables. Sometimes the tables have one lateral edge or 

 more truncated. In one fragment of a crystal I observed a 

 very obtuse acumination on the table, which appeared to be 

 diedral, the sides being placed on the obtuse lateral edges of 

 the tables. On account of the extreme frangibility of the 

 crystals, it is certainly extremely difficult to seize their cha- 

 racters. Specific gravity only 2.333, probably owing to 

 interstices between the tables. The colour is white, trans- 

 lucid, passing to semi-transparent ; lustre sometimes dull, .at 

 others shining. The tables are sometimes so aggregated that 

 their edges being exposed, offer wedge-shaped and stelliform 

 figures. The tourmalines are chiefly contained in this vein. 

 They are red, or green, rarely blue or black. 



The green tourmalines vary from one-eighth of an inch to 

 one inch in diameter ; they are sometimes four inches ia 

 length, and are entirely confined to the inner vein of quartz. 

 They are triedral prisms, with convex faces, striated longi- 

 tudinally, and generally traversed perpendicularly to the axis, 

 with very small fissures filled by some silicious substance, 

 probably feldspar. These green crystals are opaque. The 

 red tourmaline is frequently enclosed in the green. In certain 

 parts of the vein almost every green crystal encloses a red 

 one, which always corresponds by its sides and angles with 



Vol. I. ...No. 4. 28 



