MINERAL CONSTITUENTS OF ROCKS. 59 



4. The Metal-bearing Minerals or Ores, common in Rocks. — Pyrite, a com- 

 pound of sulphur and iron, in the proportion of 53 - 3 to 46 - 7, and having a very pale 

 brass-like color, much less yellow than copper pyrites; it is unlike the 

 latter also in striking fire with a steel, whence the name, from the 

 Greek for fire,. Occurs often in cubes like Fig. 57. The striae of the 

 adjoining surfaces, when any are present, are at right angles with one 

 another. Another compound of sulphur and iron, called pyrrhotite, 

 contains 40 per cent, of sulphur to 60 of iron, and is soft like the fol- 

 lowing species, but is of a pale bronze color. 



Ckalcqpyrite, or Copper Pyrites. — A compound of sulphur, copper, 

 and iron, of a deep brass-yellow color, easily scratched, and yielding a dark green pow- 

 der (and thus distinguished from pyrite); and when a solution is made with dilute nitric 

 acid, a blade of iron put into it becomes red from a coating of copper. 



Galenite, or Galena, the most common ore of lead. A compound of sulphur and lead 

 in the proportion 13'4 to 86 - 6, of a lead-gray color, soft and brittle. It occurs in cubes, 

 dodecahedrons and other forms, and cleaves easily into cubes. 



Blende (sphalerite), a compound of sulphur and zinc, in the proportion of 33 to 67; 

 of resin-yellow and brown colors, also black, and sometimes looking metallic, but giving 

 a whitish powder. Crystalline masses cleave easily, yielding rhombic dodecahedrons. 



Hematite or Specular Iron, Magnetite, and Limonite are the more common oxyds of 

 iron occurring as ores. 



Hematite, or specular iron ore (Fe 2 3 ), is often in dark steel-gray crystals or masses, 

 and also in deep-red earth y masses, and has a red powder. Magnetite (Fe 8 4 ) is in 

 dark iron-gray crystals (often octahedrons or dodecahedrons), and also massive, and 

 has a black powder. Limonite (2Fe' 2 3 +3H 2 0) occurs black and also in brownish-yel- 

 low earthy masses, and is distinguished by a brownish yellow powder. Menaccanite, 

 or Titanic iron, is an ore like hematite in its crystals, but blacker in color, and black 

 in its powder. It contains titanium as well as iron and ox} r gen. 



Graphite, called also Plumbago, and Black Lead (the material of lead pencils), looks 

 like a metallic substance; but it is simply carbon, neither lead nor iron occurring in the 

 pure mineral. 



7. Materials of organic origin. 



The materials of organic origin — that is, those derived from plants 

 or animals — may be arranged in four groups. 



(1.) The calcareous, or those of which limestones have been formed: 

 namely, corals, corallines, shells, crinoids, etc. The specific gravity of 

 corals is 2*4— 2-82 ; of shells, 2 - 4-2-86, — the highest from a Chama 

 (Silliman Jr.). 



(2.) The siliceous, or those which have contributed to the silica of 

 rocks, and may have originated flint: namely, (a) the microscopic sili- 

 ceous shields of the infusoria called Diatoms (p. 135), which are now 

 regarded as plants ; (b) the microscopic siliceous spicula of Sponges 

 (p. 132) ; (c) the microscopic siliceous shells of Poly cystines, a kind of 

 minute animal life ; id) the minute teeth of Mollusks. 



(3.) The phosphatic, or those which have contributed phosphates, 

 especially phosphate of lime ; as bones, excrements, the shells of Lin- 

 gulce, Discince, and a few other mollusks, and those of crustaceans and 

 insects, as well as ordinary animal tissues ; also the stems, leaves, and 

 fruit of plants, — especially the edible grains. Fossil excrements are 



