224 Decomposition of Iron Pyrites. 



DESCRIPTION OF PLATES YIII and IX. 



Photomicrographs of etched surfaces of pyrite- crystals by reflected 

 lamp-light (Photogelatine prints). 



Plate I. 



Fig. 1. Pyrite from Pinal County, Arizona. Mammillated or shagreen- 

 like etching, with pits, magnified 30 diameters. 



Fig. 2. Pyrite from Gilpin County, Colorado. Surface pitted in rings, 

 with small mammillations. x 23. 



Pkfte II. 



Fig. 3. Pyrite from French Creek, Chester County, Pennsylvania. Larger 

 octahedra projecting above the more deeply etched, fine, crystalline 

 groundmass. x 23. 



Fig. 4. Impure pyrite from Mahanoy City, Schuylkill County, Pennsyl- 

 vania. Minutely fretted cellular surface, without crystals or oolitic 

 structure, x 23. 



