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PROCEEDINGS OP THE AMERICAN ACADEMY. 



indistinguishable from one another. They are the same as those on the 

 corresponding crystal-face and are, so far as observed, uniform in charac- 

 ters for all strengths of acid and times of exposure. This does not imply 

 that there are no differences in the figures, but these are so minute as, in 

 most cases, to defy measurement. Since I obtained the largest and best 

 pits on actinolite, I shall call theirs the " actiuolite " type of etch-pit. 



In the diagrams (Fig. 1), (Plate I. Fig. 2), and Photograph 1, it will 

 be at once seen that in the claw-shaped figure, there are not many ele- 

 ments which can permit of precise measurement and of the comparison 

 of one figure with another, and with those of the aluminous amphiboles. 

 Yet the shape is so constant as to render possible an immediate recogni- 

 tion of these figures and of their orientation. The etch-faces are gen- 

 erally three in number (sometimes four, as in Plate I. Fig. 4), one plane, 

 the others more or less curved. The drawing and photograph convey a 

 far better idea of the arrangement of the etch-faces than could be given 

 in a verbal description, and it will only be necessary to note a few special 

 points. First, we have the corner at A (Fig. 1), which is always the 

 most clearly defined part of the figure. Often in the initial stage of 

 development this re-entrant angle appears before one can make out any- 



