i833. 139 Trans. N. V. Ac. Set. 



cretionary, and similar to those found in the clays of Ohio, the 

 Connecticut Valley, etc. A peculiarity of this mineral is the readi- 

 ness with which some specimens oxidize, while others, apparently 

 similar in all other respects, remain brilliant. Few more beautiful 

 minerals ever enter a collection ; but many of the specimens of 

 pyrite in the cabinet of the School of Mines, particularly those 

 from Schoharie, N. Y., decomposed rapidly, absorbing oxygen and 

 water, thus forming sulphuric acid, which has destroyed labels and 

 trays, and has even cut through the bottom of the drawer in which 

 they were placed. Others, like those from Roxbury, Conn., and 

 from certain gold mines of Colorado, have remained unchanged, 

 though for years lying in a room over the chemical laboratories. No 

 facts in chemical geology were more interesting and mysterious 

 than those connected with pyrite : such as its close companionship 

 with gold, the conditions of which have not been determined ; its 

 unchangeableness in some cases, its destructibility in others. Some- 

 times its crystals or concretions are completely changed to limonite, 

 with not the least change of form or markings; sometimes, by 

 oxidation, it is converted into sulphate of iron, which is washed 

 away, leaving cubical cavities, or a spongy mass of quartz ; and 

 sometimes even the iron has disappeared, leaving the cavities lined 

 with sulphur. These differences have not yet been satisfactorily 

 explained, and they constitute an inviting subject of investigation 

 for the chemist and mineralogist. 



Dr. Newberry had noticed that the pyrites so common in coal, 

 and pyrite replacing wood, are particularly prone to oxidation, the 

 concretions in clays liable to it, and the briUiant crystallizations in 

 mineral veins and in metamorphic slates less so. 



Prof D. S. Martin observed that these specimens do decom- 

 pose with wonderful readiness at Keyport, and in a peculiar way, 

 where exposed to the salt water, the ordinary alteration into sul- 

 phate of iron being there replaced by a change into limonite 

 apparently through some action of the sea-water, the pebbles in the 

 vicinity being also encrusted by limonite. 



The Corresponding Secretary read letters from Dr. Weiss- 

 bach, of Freiburg, Prof. Jannetaz and M. Emile Bertrand, of 



