164 Decomposition of Iron Pyrites. 



c)f some of them })hoto-microgTai)lis are presented in the two- 

 Plates illustrating this ])aper. 



a. {Plate 8, Figs. 1 and 2). A mammilhited or a i)itted sur- 

 face, more or less uniform, best compared to that of shagreen in 

 miniature. The pitting alone, at first glance, might suggest the 

 regular ei'osion of a homogeneous material iiito rounded cavities, 

 as it progressed uniformly from separate scattered points. A 

 distinctly oolitic structure, hovvevei-, is shown on the mammil- 

 hited surfaces, by the projecting i)apill8e, or the pits left by their 

 removal, which plainly correspond to s})herules, 9 to 82 micrasin 

 diameter, which vary, however, considei'ably in form, being often 

 elliptical, rectangular, or represented by cubes. They usually 

 appear to be constituted of exceedingly minute granules, 3 to 6 

 micras in diameter, sometimes spherical in form, often cubical ; 

 these minute cubes occasionally show a diffei'ent orientation of 

 their axes from each other and from the main crystal, as if 

 grouped concentrically around scattered nuclei. This shngreen- 

 like surface is predominant on most of the etched faces of the 

 heavier crystals of pyrite, down to the specific grnvity of 4.98. 

 The only a})parent exception, thut of Specimen No. 3. at the 

 head of the Table, is the one whose density — as we have already 

 had evidence — is probably unduly increased, partly by enclos- 

 ure of galenite. I therefore believe this oolitic sti ucture to be 

 characteristic of normal pyrite, tind this shiigreen-like surface, 

 when general over an etched ciystal, to bear testimony to the 

 purity and stability of the mineral. 



h. {Plate 9, Fig. 3). A surface roughened by coai'se irregular 

 projections above an even plane, very finely granular and some- 

 times shagreen-like. The structure here consists of coarsely 

 crystalline yellow grains, sometimes octahedra or cubes, of })ure 

 pyrite, resisting oxidation, scattei-ed in greater or less number 

 through a fine grained and impure groundmass, which may be 

 lighter in color and yields more readily to oxidation. This het- 

 erogeneous structure may occur in some of the heavier varieties, 

 but is characteristic of those with a specific gravity below 4.98 

 and with inferior stability. 



c. A surface either ribbed by projecting parallel yellow ridges, 

 marking the cleavage planes, or furrowed by lighter colored 

 clefts and lines which have been etched out. In these we have 



