l886.J NEW-YORK MICROSCOPICAL SOCIETY. 87 



original amount of iron. Thus, marcasite is born, consisting of 

 iron di-sulphide, Fe* S*, or more simply expressed, Fe S^. This 

 is a steely-grayish white mineral, often affected by a greenish 

 tinge, crystallizing after an entirely different fashion, the rhom- 

 bic, and having a peculiar tendency to originate in calcareous 

 rocks ; e. g., the chalk of England and the Carboniferous and 

 Trenton limestones of this country. From a pound of the latter 

 rock from Jefferson County, New York, digested in acid, 

 over an ounce of marcasite, in drusy crusts of pure and 

 brilliant crystals, was left as a dark se'diment. A glance at the 

 second slide will convey a sufficient idea of the steely color, the 

 curious involved crystallization, and the deeply striated faces 

 which commonly characterize this mineral. 



You will notice also, in the same slide, here and there, deli- 

 cate white needles in radiating groups, planted on the surface of 

 the marcasite crystals. These consist of "copperas," or "white 

 vitriol," /. e., sulphate of iron, and are the first evidences 

 of the attack and absorption of oxygen from the air. 



But in the next slide, the marcasite from Galena, Illinois, we 

 have a striking example in miniature of that iridescent tarnish, 

 examples of which are to be found in the cabinet of every 

 mineralogist, a gaudy display of color, which, like that of the 

 autumnal foliage of our forest, is only a proof of weakness, of 

 oxidation by the air, and of coming dissolution, but in the case 

 of the mineral, with no hope of the re-birth and of the new term 

 of life which follows in the case of the tree. 



The last member of the triad of the pyrites is Pyrite, which is 

 found in brass-yellow cubes or octahedra, breaks with a con- 

 choidal fracture like porcelain, and yet has exactly the same 

 composition as marcasite, Fe S^. In general, it is also found at 

 the last stage of the alteration of crystalline rocks, where a high 

 temperature has prevailed, as in the gneiss and magnesian mar- 

 bles of New York Island. In its pure form, which almost de- 

 fies oxidation, you may see it in the polished faces of this cube 

 from Chili, in which you will notice that the microscope only 

 confirms the ocular evidence of its density and purity. But here, 

 on the other hand, is a granular form of the mineral from New 

 Jersey, decaying so fast that the formation and expansion of the 

 vitriol crystals within it are rending it to pieces. In the next 

 slide, a fragment of this granular mass presents to the eye an 



