Blasdalk.] 



Contribution to Mineralogy. 



331 



The general features of the etch-pits thus obtained are shown 

 in the accompanying figures. No. 1 represents a partly- and No. 2 

 a fully-developed pit, No. 3 represents a group of pits one of 

 which has attained its perfected form. The essential character of 

 all the normally-developed pits consists of three curved faces, 



Fig. I. Fig. '2 Fi 8' 3. Fig. 4. 



which form a solid angle near the center of the pit, and intersect 

 the crystal plane in three lines, two of the latter being straight or 

 very nearly so, and of nearly equal length, the third forming a 

 long bow-shaped curve. One of the straight lines forms a slight 

 angle with the prismatic cleavage trace ; the other, commonly the 

 longer, is at a considerable angle with it. In general the figures 

 closely resemble those obtained with normal actinolite by Daly, 

 the variations, such as are observed in the curvature of the longest 

 line of the figure, being no greater than those obtained in etching 

 different crystals from the same locality. For the angle between 

 the line AB and the cleavage trace Daly obtained values varying 

 from —2° to +12 according to the stage of development of the 

 figure; the present material gave members ranging from 4 to 

 8° 15'; for the angle between AC and the cleavage trace Daly 

 obtained from io° to 16 , while the material under consideration 

 gave from io° to 16 36'. 



The absence of terminal planes in actinolite crystals renders it 

 impossible to correlate with certainty the position assumed by the 

 etch-figures with reference to the crystallographic orientation. A 

 study of the positions assumed by the figures on the four prismatic 

 planes gave results in accord with the orientation adopted by Daly. 



