166 Decomijosition of Iron Pyrites. 



der trial, as ensured by preliminary microsco])ical examination ; 

 the projoer mode of experiment, {. e. , on coarsely crnshed 

 material, rather than on complete crystals ; the exact crystalline 

 form, as an assurance of identity ; and any data connected with 

 the tendency to decomposition of each specimen under investiga- 

 tion. I have now to present the results of a careful examina- 

 tion of a large number of specimens^ of iron-pyrites in my own 

 cabinet, from American and foreign localities, chiefly crystals 

 selected for their perfect forms and apparent purity. A brief 

 description is first given of every specimen in the collection of 

 each of the three minerals, concluding with the apparent evi- 

 dences of visible decomposition. In the tables, the collection- 

 number of each specimen is given in the first column, and its 

 locality in the second. The specimens of each mineral are num- 

 bered and arranged in the order of decreasing specific gravity, 

 stated in the third column. A large part of these determina- 

 tions was made, with the help of two assistants, on material 

 which I had previously selected, crushed, picked out and exam- 

 ined under the microscope. In most instances, the specimens, 

 crushed to coarse powder, were previously fi-ecd from the film 

 or stain of adhering iron-oxide by short digestion in dilute 

 hydrochloric acid ; this was thoroughly washed out, before the 

 specific gravity determination was taken. The exceptional cases, 

 in which this pieliminary digestion in acid was omitted as un- 

 necessary, are indicated by an asterisk, attached to the collec- 

 tion-number of the specimen in the first column. In the fourth 

 column is recorded the actual weight in grams of the amount 

 of mineral used in the preceding determination, as one element 

 of judgment of its accui-acy. In the tables for marcasite and 

 pyrite, other columns present my view (explained beyond) of 

 the theoretical mineralogical constitution of each specimen ; 

 and in the table for pyrite, the prevailing ci-ystallographic forms 

 are presented in the sixth column. At the close of each prelim- 

 inary description, as well as in the last column of every table, 

 are recorded the evidences of decomposition apparently exhibit- 



^ My grateful acknowledgements are due to Profs. B. K. Emerson, T. 

 Egleston, J. S. Newberry, and D. S. Martin, and Messrs. B. B. Chamber- 

 lin, J. M. Habirshaw, and B. G. Amend for the contribution of thirty 

 specimens for this investigation. 



