l886.] NEW-YORK MICROSCOPICAL SOCIETY. 93 



flattened out by intense lateral pressure, whose irregularity had 

 produced the wavy disturbance of their lines ; also, that their 

 material was harder and perhaps brighter than that in the inter- 

 vening furrows, and that their projection above the surface was 

 but a feature of the general erosion, caused by their greater resist- 

 ance to decomposition than that offered by the intervening films. 



4. The suspicion of the want of homogeneity of the material 

 was increased by the occasional distribution, over the dull eroded 

 surface, of minute bright grains, scales, pellicles, and angular, 

 sometimes branching rods, contrasting with the general surface 

 by their brilliant lustre, yellow color, and sometimes, a slight 

 projection. Many consisted of barely visible particles and lines, 

 but some were noticed of the size of 0.005 to 0.035 '^'^'^• 



Occasionally, triangular and rectangular outlines 0.056 mm. 

 in length, and rarely two or three faces of a flattened polished 

 cube could be distinguished. Their compact, bright material 

 appeared identical with that of the striation films, and, in fact, 

 many of their forms appeared as mere expansions or projec- 

 tions of these films. All the facts strengthened the idea that 

 the material of this fibrous pyrite is not uniform, as it appears 

 to the eye, but that these grains and minute lines indicate the 

 planes of successive envelopment of two materials, the one 

 yielding rapidly to decomposition, the other more dense and 

 yielding more slowly. 



The little pits or cavities were also closely examined to deter- 

 mine whether they ever presented symmetrical outlines, which 

 might signify the eating away of crystals of a softer substance, 

 but no such indications were recognized. 



D. The darkened surface of a cube from the outer surface, 

 slightly marked to the eye by minute particles of the vitriol- 

 efflorescence. This presented under the microscope a finely 

 granular mass of the pyrite material, seamed and interspersed 

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

 peras in an almost continuous network. The surface was, in 

 general, deeply and very irregularly eaten out and honeycombed 

 in pits and cavities divided by jagged angular septa; these 

 cavities often presented a diameter of 0.06 to o.io 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 in- 

 dicated the full development of the cubical cleavage. 



