Dccu}np()sitio)i of Iron ]\f/riJes. 145 



gnmiilar mass of ilie ])yrite-miiterial, seamed and interspersed 

 Avith the white particles, grains, and sometimes needles of cop- 

 peras in an almost continuous network. The surface ^vas gen- 

 crall}' deeply and vei'v irreguhirlj eaten out and honeycombed in 

 pits and cavities divided by jagged angular septa ; these cavities 

 often possessed a diameter of 0.06 to 0.10 mm., with a depth of 

 about the same amount. Many of the grains of pyrite displayed 

 cubical outlines, about 0.025 mm. on a side, which indicated the 

 full development of the cubical cleavage. On similar corroded 

 and effloresced surfaces, on the side of fissures further down, be- 

 low the point where the cubes were developed, the eroded pits 

 were a little larger, about 0.167 mm. in diameter, but the grains 

 of pyrite did not exhibit cubical forms. 



Py rife, from Chili. The polished surface of a cube, about 1 

 centimeter on a side. To the eye, this surface showed a dis- 

 tinct striation in places ; under the microscope, little more than 

 jdanes of accretion, parallel to the edges of the cube, became 

 visible. The material appeared almost perfectly uniform and 

 homogeneous. 



Pyrite, from WeehawJcen, N. J. The surfaces of octahedra, 

 in various stages of incipient oxidation. K surface which was 

 brilliant and polished to the eye appeared almost equally so 

 under the microscope, and the underlying material, shown on 

 conchoidal fractures alono- the edsfes, seemed to be homo- 

 geneous, though very pale in color. But many other surfaces, 

 showing a tarnish to the eye, presented in high illumination, 

 under the microscope, a brilliant display of iridescence, re- 

 sembling that of marcasite. Very often this tarnish was not 

 uniformly distributed, but displayed alternating bands of blue 

 and red colors, parallel, or intersecting each other in two or 

 three directions, parallel to the edges of the octahedral faces. 

 This mode of decomposition implies probably a uniform distri- 

 bution. of an enclosed unstable impurity ; since the course of the 

 oxidation has conformed exactl}^ to the cleavage structure of the 

 crystal. Other octahedra from the same locality showed the 

 last stage of complete oxidation, in the form of limonitic pseudo- 

 morphs, preserving, almost perfectly, the lustre of the faces and 

 sharpness of the edges and minute modifications. In i^laces, 

 delicate reddish-brown crusts were found deposited upon some 



