644 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



crystals measured. Only three of the observed faces were cleavage 

 planes, one each on three different crystals. Some of these faces were 

 smooth with a few hair-like striations on them, but for the most part 

 the form is striated considerably from oscillatory combination with the 



Table of observed Measurements on Diaspore. 



prisms. On crystals of the slender prismatic habit its faces are neces- 

 sarily narrow, broad on the disc-like crystals and medium on the stout 

 habit. It is the dominant form on the Chester diaspore, and perfect 

 cleavage parallel to it is characteristic of the species. 



a (100). Only two faces of this form were observed as line-like 



