88 JOURNAL OF THE [J^^nC, 



aggregate of little bright cubes of pyrite, but no apparent reason 

 for this speedy destruction. To solve this difficult problem of 

 the varying stability and instability of crystals of the very same 

 mineral, I ask your attention next to the octahedral crystals of 

 pyrite from Weehawken, New Jersey. You will notice that the 

 faces of these are covered by a more or less marked iridescence, 

 reminding you of that already shown on the marcasite crystals 

 of Galena, Illinois, although no visible trace of that mineral can 

 here be detected. It is significant, also, that, in these crystals 

 of pyrite, the tarnish is not uniformly dispersed over the sur- 

 face, but very often presents alternating narrow stripes of color, 

 blue or green with red. These lie sometimes in a set parallel to 

 one edge of a triangular face, sometimes in two or even three 

 intersecting sets, parallel respectively to the three edges of a 

 face of the octahedron. This implies that the oxidation of the 

 material has not progressed uniformly, and can hardly be ex- 

 plained except on the supposition that the pyrite encloses some 

 unstable impurity, arranged in correspondence to crystallograph- 

 ic symmetry. The mineralological microscopist will recall many 

 analogous instances of a similar internal arrangement of im- 

 purities, caught up during crystallization, such as those in the 

 cruciform macles of chiastolite from Lancaster, Massachusetts. 

 In these bright pyrite-crystals, it is true, the secret enemy, which 

 effects their destruction on weathering, is diffused in a form far 

 more minute, almost one of molecular isolation. But although 

 the distinction of its particles exceeds all the powers of the 

 microscope, its presence is here clearly revealed, as I believe, 

 through the play of chemical forces and the subtile testimony of 

 light. 



The oxidation of the Weehawken pyrite, however, does not 

 stop with this iridescent tarnish. The last change of all is 

 shown in its complete alteration into a liver-colored limonitic 

 iron ore, and yet, as displayed in the next slide, with the almost 

 perfect preservation of the surface polish, and of the sharpness 

 of its edges. Some mystery has always been attached to the 

 origin of such hepatic pseudomorphs of iron oxide, presenting 

 the crystalline form, properly belonging to pyrite. Your atten- 

 tion is further invited, therefore, in these Weehawken pseu- 

 domorphs, to the delicate reddish-brown crusts, deposited upon 

 some of the crystal faces. On a cross-section, these crusts show 



