156 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



isometric habit, bounded by the forms Poo , Poo , ooP, P, and wPoo . 

 Fig. 5a. Orthographic projection of a polysynthetic trilling of marca- 

 site of type 5 on the basal pinacoid. Fig. 6. Twinned parallel growth 

 of marcasite of type 6 bounded by the forms ooP, Poo , and Poo . Fig. 6a. 

 Orthographic projection of marcasite trilling of type 6 on the basal pin- 

 acoid. Fig. 6b. Similar projection of simple twin of same type of mar- 

 casite. Fig. 7. Crystal of marcasite of pseudo-octahedral habit (type 7) 

 bounded by the forms Poo , Poo , OP, oo P, and P. Fig. 8. Crystal of pyrite 

 from Shullsburg. Fig. 9. Pyrite aggregate from Mineral Point. 



Mineralogical Petrographical Laboratory, 



University of Wisconsin. 



