ALUMINA. 335 



coal, it melts with difficulty into a blebby semi-transparent glass ; with borax, it dissolves 

 into a clear globule. It does not give out water by calcination, and is not acted on by acids. 



Varieties. Common Feldspar. This is mostly opaque, or translucent only on the edges. 

 It is of various colours, and is either crystallized or massive. It is very widely diffused, as 

 it forms one of the constituents of granite. 



Adularia, sometimes called Moon-stone. A semi-transparent or translucent variety, which 

 is white, greyish or milk-white, and is frequently iridescent. It sometimes has a pearly lustre, 

 and exhibits, when cut and polished, a bluish or greenish white chatoyant reflection, whence 

 the name moonstone is derived. 



Glassy Feldspar. This includes the semi-transparent varieties. It has a vitreous lustre, 

 whence its name. It occurs generally in crystals which have the appearance of being cracked 

 in various directions. This is considered as a distinct species by Dr. Thomson. 



Ice Spar. Occurs in white, transparent or translucent flattish crystals. 



Porcelain Clay, or Kaolin. This is a decomposed feldspar, which is massive and dissemi- 

 nated in certain rocks, and is composed of small particles which possess but slight coherence. 

 It adheres to the tongue, and is soft and meagre to the touch. Crystals of feldspar, mica, 

 etc. are sometimes found in it. 



Necronite. This variety is characterized by giving out an offensive odour on being rubbed 

 or struck, but it does not differ in other respects from common feldspar. 



Composition. Adularia — Silica 64.24, alumina 18.40, potassa 16.95, lime trace {Ber- 

 thier). 



Lamellar Feldspar (red) — Silica 65.03, alumina 17.96, potassa 16 .21, lime 0.35, peroxide 

 of iron 0.47 {Beudant). 



Green Feldspar — Silica 62.85, alumina 17.02, potassa 13.00, lime 4.00, oxide of iron 

 1 .00 {Vauquelin). 



Geological Situation. Feldspar is one of the most abundant minerals, and it occurs in 

 most of the primary rocks. The finest specimens in this State are in or near the white lime- 

 stone. 



Uses. Some of the varieties of compact feldspar are susceptible of a fine polish, and are 

 esteemed by the lapidary. They are also used in the manufacture of porcelain. 



localities. 



Dutchess County. Ferruginous green feldspar, containing granular epidote, is said to 

 occur in Rhinebeck. 



Essex County. At Rogers' rock, adularia and common feldspar are found associated with 

 various other minerals. Sometimes it is in minute transparent crystals in a loosely aggre- 

 gated mass, associated with garnet and small crystals of calcareous spar. The crystals 



