Decomposition of Iron Pyrites. 217 



stone. Of this several instances have been brought to my 

 notice, in masonry within New York City. The gray biotitic 

 gneiss of the island, in large part pyritiferous, has been much 

 used, not only for foundations but for basement-walls up to the 

 water-table, and even for the faQade of large public edifices. 

 The rock often contains a notable amount of pyrites, sometimes 

 in the form of polished octahedra of pyrite/ or crusts of marca- 

 site, most commonly in the form of scattered grains and flakes 

 of pyrite ; this gneiss has yielded 0.67 per cent, of sulphur on 

 analysis." Where such material has been used without selection, 

 in masonry, and exposed to the weather above the ground-line, 

 an irregular and offensive dirty reddish brown blotching has 

 taken place, after weathering but a few years, e. g., in the re- 

 taining-walls of the enclosure at 51st to 52d streets and Madison 

 avenue, the basement of the hospital at 71st street and Madison 

 avenue, etc. In the natural outcrops of the gneiss, the results of 

 oxidation commonly consist of a white vitriol, alum, and hy- 

 drated iron-oxide {e. g., at 60th street and 11th avenue, 72d 

 street and Avenue B, etc.), or even of limonite pseudomorphs 

 after pyrite-cubes, which I have found at 120th street and St. 

 Nicholas avenue. 



This pyrite is nearly white on fresh fracture, sometimes asso- 

 ciated with crystallized marcasite (Marcasite No. 13), in its 

 natural outcrops over the island. An inferior variety of the 

 light buff -colored sandstone, from the Lower Carboniferous of 

 Nova Scotia, was seen in the ashlar of a row of buildings at 87th 

 street, near 2d avenue, spotted with large reddish brown nod- 

 ules of pyrite in active decomposition, apparently identical 

 with the nodules described under Pyrite Nos. 80 and 84, with 

 Sp. Gr. 4.909. In the dolomitic white marble of the island and 

 Ticinity of New York, the pyrite is sometimes quite stable, \vith 

 Sp. Gr. 5.003 to 4.998 (Pyrite Nos. 23 and 29), and in other 

 beds readily passes into hepatic decomposition having Sp. 

 Gr. 4.946 (Pyrite No. 23^). In most of the limestones and 

 marbles of Vermont and of the Housatonic valley through 

 Massachusetts and Connecticut, the pyrite belongs to the quick- 

 ly perishable variety, and the rock in its vicinity soon becomes 



» S. C. Bailey. Ann. Lye. Nat. Hist.. N. Y.. (1865), YIII, 190. 

 ^ P. Schweitzer, Jhrsb. Chem.. (1878), 1282. 



