THE NONMETALLIC MINERALS. 191 



disseminated grains throughout the mass of a rock, or along the line 

 of contact between basic eruptivesand sedimentaries; as irregular and 

 sporadic and concretionary masses in sedimentary rocks and modern 

 sands and gravels; in the form of true fissure veins, and as interbedded, 

 often lenticular masses, sometimes of immense size, lying conformably 

 with the stratification (or foliation) of the inclosing rock. On the 

 immediate surface the mineral is in most cases considerably altered by 

 oxidation and hydration, forming the caps of gossan or limonite. 



The origin of the mineral in the older crystalline rocks, as that of 

 the rocks themselves, is not infrequently somewhat obscure. In sedi- 

 mentary rocks it is undoubtedly due to the precipitation of the included 

 ferruginous matter by sulphureted and deoxidizing solutions from 

 decomposing animal and vegetable matter. 



Some of the pyritiferous deposits, as those of Louisa County, Virginia 

 (Specimens Nos. 54239, 54241, and 54242, U.S.N.M.), and Huelva. Spain, 

 are of enormous proportions. The first named is described 1 as over 

 2 miles in length, and to have been exploited to upwards of 600 feet in 

 depth and in width, from foot to hanging rock, as high as 60 feet of 

 pure ore (see large Specimen No. 54242, U.S.N.M.). The average 

 width of the two worked beds is upward of 18 feet. The rocks 

 inclosing the deposits consist principally of talcose and hydromica 

 slates. At Rio Tinto the ore is described 2 as occurring in immense 

 masses several thousand feet in length and from 300 to 800 feet in 

 width, extending in depth to an unknown distance. The ore (Specimen 

 No. 11427, U.S.N.M.) is very clean and massive, containing besides 

 sulphur and iron only some 2 to 4 per cent of copper and traces of 

 silver and gold. The material is mined wholly from open cuts and 

 to a depth of some 400 feet. The country rock is described as of 

 Silurian and Devonian schists near contact with diorites. 



Uses. With the exception of the small amount utilized in the prep- 

 aration of vermilion paints and the still smaller amount used for 

 jewelry, almost the sole value of the mineral is for the manufacture of 

 sulphuric acid and the sulphate of iron, known as green vitriol or cop- 

 peras. In the process of making sulphuric acid the ore is roasted or 

 burnt in specially designed ovens and furnaces until the mineral is 

 decomposed, the sulphur fumes being caught and condensed in cham- 

 bers prepared for the purpose. By the Glover and Ga^v-Lussac method 

 from 280 to 290 parts of sulphuric acid of a density of 66 Baume may 

 be obtained for each 100 parts of sulphur in the ore or about 2,565 

 pounds of acid to 1 ton (2,000 pounds) of average ore. 



In the manufacture of copperas the ore is broken into small pieces 

 and thrown into piles over which water is allowed to drip slowly. A 



1 Origin of the Iron Pyrites Deposits in Louisa County, Virginia, by F. L. Nason, 

 Engineering and Mining Journal, LVII, 1894, p. 414. 

 - A Visit to the Pyrite Mines of Spain, Eng. and Min. Jour., LVI, 1893, p. 498. 



