140 



The Monoclinic Pyroxenes of New York State. 



The section giving a perfectly symmetrical and satisfactory 

 interference figure was obtained at every trial. The smallest 

 crystal sectioned was three-eighths of an inch long and the same 

 in width. 



Etchfigukes. 



Crystals of diopside from DeKalb and augite from Piteairn, 

 showing smooth and bright faces, were treated with warm hydro- 

 fluoric acid for several minutes, and in both cases with the follow- 

 ing results : 



On the prism face there was produced an acute angled triangle, 

 whose longer side was parallel, or very nearly so, to the edge 

 110 A 010. The acute angle always pointed towards the positive 

 hemipyramid, while the obtuse augle pointed towards the ortho- 

 pinacoid. 



On the orthopinacoid the etchfigures were irregularly deltoid 

 in outline, with the lower end rounded, and the upper end always 

 drawn out into a point which was towards the negative hemi- 

 pyramid. 



Somewhat irregular figures were produced on the clinopinacoid, 



