406 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



of etching on the clinopinacoid. Good figures are obtained with ease. 

 (Plate I. Figs. 23a, 236, 23c.) They are remarkable in belonging to 

 two classes, analogous to those described by Pelikan for pyroxene with 

 the same reagent.* (Photo. 12, cf. Photo. 13.) The pit of the one 

 category is a quadrilateral in outline, with four pyramidal figure-faces 

 and a bottom-face that grows smaller and then disappears as the figure 

 matures and deepens. The other kind of pit is also four-sided in habit, 

 but may possess another pair of figure-faces in addition to the five cor- 

 responding to those of the first class. Figures 5 a and 5 b represent 

 diagrammatically the two kinds matured under normal conditions (water 

 bath, concentrated acid, etc.). The drawings are lettered in order to in- 

 dicate the elements chosen to fix the shape and orientation of the pits. 



Figure 6. 



Figure 5 6, 



C Figure 7. 



* Ueber den Scliichtenbau der Krystalle. Min. und petrog. Mittheil., 1896, 

 Bd. XVI. p. 16. 



