94 JOURNAL OF THE [June, 



On similar corroded and effloresced surfaces, on the side of 

 fissures below 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. 



From these observations we may gather : — 



1. In these fibrous nodules of pyrites, the fibres consist of 

 elongated cubes, whose outward growth and mutual compres- 

 sion have produced a condition of great tension. 



2. The material consists mainly of a diluted mixture of pyrite 

 with a paler-colored and unstable impurity. Through this mix- 

 ture more or less pure pyrite is diffused in alternating films, or 

 in scattered strings and crystals of deeper-yellow color than 

 their matrix. 



3. The oxidation of the material has been facilitated by its 

 heterogeneous composition, by its fissured structure, and by the 

 tension among its fibres. It has progressed more rapidly in the 

 predominant pale-colored mixture, has penetrated along the 

 seams between the fibres, and has then been hastened by the 

 development of the more minute Assuring as the result of the 

 tension. 



4. The development of this system of minute fissures has fur- 

 nished an enormous area for the internal condensation of gases 

 and vapors from the atmosphere, chiefly oxygen and moisture, 

 which has resulted in the speedy oxidation, pitting, decay, pro- 

 duction of crystals of vitriol, expansion, and final disintegration 

 observed in such forms of pyrites. 



A consideration of all the facts leads, I think, to the conclu- 

 sion that the readiness to decompose in certain forms of pyrite 

 is connected, only to a slight extent, with the nature of the sur- 

 faces of its crystals, and but in part with any visible internal 

 peculiarities of structure. We may rather be inclined to infer a 

 want of homogeneity in unstable pyrite, produced by an intimate 

 intermixture with some unstable though invisible imi)urity, and 

 analogy leads us to attribute the latter to the presence of some 

 proportion of marcasite. 



I have so far brought before you to-night the various peculiar- 

 ities of texture revealed by the microscope in the crystals and 

 nodular forms of the three pyrites, and have exhibited in minia- 

 ture, in the slides, very nearly all the modes of decomposition 

 by which these minerals are attacked. Some new facts of the 



