14-2 Decomposition of Iron Pyrites. 



this purpose low magnifying powers, up to 200 dicimeters, were 

 found sufficient, Avith the help of the plane mirror of a Sorby 

 reflector. The following materials were thus examined. 



a. A fibrous plate of the fresh and brilliant material from the 

 interior. The surface of this natural fragment was divided up 

 by the fibration, by strongly marked lines, sometimes perhaps 

 indicating open fissures, 0.033 to 0.134 mm. apart. Within 

 these in many places a still finer lineation occurred, the lines 

 being sometimes only 0.014 mm. apart. These finer lines, co- 

 inciding with the cubic cleavage, were sometimes parallel to 

 the main fibration, sometimes perpendicular to it ; elsewhere, 

 very commonly arranged obliquely at an angle of 45° to 53° 

 from the general direction, sometimes even in two sets passing 

 obliquely off in opposite directions from a median line. These 

 latter oblique lines doubtless mark the octahedral cleavage of 

 pyrite, often greatly distorted by pressure and even thereby ren- 

 dered curvilinear. A want of homogeneity was suggested by 

 a number of bright angular yellow particles and grains, scattered 

 over the white and duller surface : their size usually varied from 

 0.013 to 0.084 mm. 



h. A fragment, from a plane at right angles to that of a, pre- 

 senting the polished mammillary and curved surface from the 

 cross-fracture. The entire surface was found to be, not uniform 

 as it appeared to the eye, but seamed and slightly roughened by 

 short fissures, marking the cubic cleavage, running at right 

 angles to each other, but rarely intersecting, and dividing up tht^ 

 surface into square spaces about 0.01 to 0.015 mm. on a side. The 

 same bright yellow grains appeared here and there, as in a, but 

 mostly as lines or thin branching veins, apparently the edges of 

 films of yellow material enclosed in the paler colored pyrite. 



c. A portion of the side of a fissure, plainly to the eye dark- 

 ened and roughened by incipient decomposition, but still appar- 

 ently perfectly dense and compact, showed under the microscope 

 a remarkable sub-division and disintegration, the whole surface 

 being seamed by minute cracks, mostly along and across the 

 fibres, and also irregularly pitted and even honeycombed with 

 cavities of the most irregular shape and size : all this surface 

 was sprinkled and coated with granules and needles of the 

 white efflorescence. The phenomena differed widely on every 



