SILICA. 259 



yellow or green, which occur massive and disseminated, with a fracture varying from con- 

 choidal to earthy, and a lustre from ghstening to dull. They are opaque. 



Spongiform Quartz, or Floatstone. It has a spongy or porous appearance, and consists of 

 numerous white or greyish white crystals. It is so porous and light as to swim on water, 

 whence its name. 



Siliceous Sinter. This is white, greyish white and yellow. It is hght and brittle, dull, 

 commonly porous, with a fibrous texture, although sometimes sufficiently compact to admit 

 of a conchoidal fracture. Lustre pearly. 



Composition. Rock crystal — Silica 99.37 (Bucholz). 



Amethyst — Sihca 97.50, alumina 0.25, oxide of iron a trace, manganese 0.75 (Rose). 



Common Quartz — Silica 97.75, alumina . 50, water 1.00 (Bucholz). 



Red Iron Flint — Silica 76.8, alumina 0.25, oxide of iron 21.66, volatile matter 1.00 

 {Bucholz). 



Carnelian — Silica 94 . 00, alumina 3 . 50, oxide of iron . 75 {Bindheim). 



Chrysoprase — Silica 96.17, alumina 0.08, lime 0.82 (Klaproth). 



Common Flinty Slate — Silica 51.84, alumina 15.43, lime 9.42, oxide of iron 9.96 

 {Dumeril). 



Geological Situation. This mineral is very widely distributed, and is one of the most 

 abundant in nature. It is found in almost every formation. In the State of New- York, the 

 finest crystalline varieties are obtained in the sandstone, slate, and calciferous sandrock. 



localities. 



As quartz is so extensively diffiised, I shall notice only the more important localities of 

 those varieties which are usually esteemed as mineralogical specimens. In this respect, our 

 State is pecuharly rich, and probably no part of the world afi'ords more beautiful or more inte- 

 resting crystalline forms. 



Albany County. In the fissures of slate in this county, transparent crystals have been 

 found. The most noted locality is Crystal hill, about three miles below the city. 



On the Helderberg, about sixteen miles from the city, druses of quartz crystals are found 

 in the limestone, of a dull yellowish colour. They were ca.\hd ferruginous quartz, but they 

 do not belong to that variety. Hornstone is also common in this rock. 



Columbia County. I have only to remark here, that at Chatham, there is an amorphous 

 mass of quartz, more than an hundred feet in diameter, with hexahedral crystals dissemi- 

 nated.* 



Essex County. Crystallized quartz is found in various parts of this county, but I am not 

 acquainted with any locality at which the specimens are remarkable for their beauty. 



Rose quartz, of a very rich and beautiful colour, is found in the vicinity of Port Henry. 



^ Eaton. Cleaveland^8 Mineralogy. 



