92 JOURNAL OF THE [J'^mc, 



C. A portion of the side of a fissure evidently — to the eye — 

 darkened and roughened by incipient decomposition, but still 

 apparently perfectly dense and compact. This showed under 

 the microscope a remarkable subdivision and disintegration, the 

 whole surface being seamed by minute cracks, mostly along and 

 across the fibres, and also irregularly pitted and even honey- 

 combed 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 surface examined, but mostly comprised the following 

 points of structure in the pyrite itself : — 



1. A coarse columnar structure, that of the fibration, present- 

 ing a width of about 0.08 to 0.25 mm. between the parallel fis- 

 sures, whose lips were separated about 0.005 mm. This was 

 crossed, with more or less irregularity, by fissures at right angles, 

 often producing the effect of an imperfect tesselated pavement, 

 or of rude masonry. In places, the disintegration had gone so 

 far that the mass consisted of dark roughened needles, attached 

 at one end, or both. 



2. The surface intervening between these cracks was pitted 

 with cavities of the utmost irregularity of size and form, though 

 commonly approximating 0.004 to 0.009 "^"^- i'^ diameter, scat- 

 tered in rows and in large groups. As a result of the subdivision 

 produced by these cracks and pits, I estimated that the greater 

 part of the mass was separated into little grains, approximately 

 cubical in form, and about 0.0 1 mm. on a side. It would re- 

 quire about a thousand millions of such little grains to make up 

 a cubic centimetre of the material, and the surfaces of these 

 would present a superficial area about ten million times that of 

 the superficies of a solid cubic centimetre. 



3. The surface between the little pits further shows a very 

 delicate striation, apparently caused by fine cracks or by minute 

 ribs and furrows, all parallel to the line of fibration, but slightly 

 wavy. On an average, about 555 of such lines occupied the 

 distance of one millimetre across the fibration ; /. <r., they were 

 about 0.0018 mm. apart. They projected very slightly above 

 the intervening furrows, and produced the impression that they 

 were lines of accretion running in the direction of the general 

 fibration, and originally producing that structure ; that they 

 represented the edges of thin films of a compact material, 



