OF ARTS AND SCIENCES. 



317 



Crystalline Plates in Galena. 



In this connection it may be of interest to describe some specimens 

 of galena which appeared among the minerals used for blowpipe 

 analysis by the class in mineralogy at Harvard College, but whose 

 origin is unknown. 



Fig. 2. 



There were found three crystals very much alike, one of which is 

 shown considerably enlarged in the accompanying sketch. On one side 

 it showed the perfect cubic cleavage of ordinary galena, but the mass 

 of the crystal consisted of a network of plates about 1 mm. thick, in- 

 tersecting each other for the most part at right angles, and thus form- 

 ing a skeleton cube. But that which makes the specimens of unusual 

 interest is the fact that there are certain additional plates, following 

 the direction of the faces of an octahedron, and thus running diago- 

 nally through the entire thickness of the crystals. Such a plate is 

 shown at A B in the sketch (Fig. 2). 



Moreover, the cleavage of the galena appeared to be due to some 

 other cause than the form of the crystal, since the octahedral plates 

 were as well marked and nearly as abundant as the cubic ones, while 

 the cleavage was distinctly cubic. Thus it appears that when galena 

 crystallizes, just as when iron crystallizes, plates form through the 

 mass following the directions of both the cube and octahedron till 



