DALY. ETCH-FIGURES ON AMPHIBOLES. 393 



thing of the rest of the figure (Plate I. Fig. 1), a phenomenoa tliat 

 seems to have its correlative in the filling of the deeper parts, first of the 

 figures on a crystal of alum when it is placed in a concentrated solution 

 of its own substance (Ausheilen) .* The edge A £ is likewise the most 

 sharply outlined edge. The angle £ A i/^ which it makes with the trace 

 of the cleavage can be seen to alter as the figure gradually evolves in the 

 process of etching. Thus, on Zillerthal actinolite, this angle increases as 

 the figure grows more mature, from minus 2° to plus 10°, plus angles 

 being read on the right of the cleavage trace passing through A (compare 

 in Plate I. Fig. 1, Fig. 2, and Fig. 3). The limits of variation are the 

 same for the Syra actinolite. In the actinolite numbered V. 14, they are 

 0" and 12° ; in V. 16, 0° and 8°. During this jirocess of swivelling, the 

 edge AB remains straight ; this suggests an actual change in the indices 

 of the figure-face adjoining AJ3 as the pit is deepened, rather than a modi- 

 fication of a figure-face of constant indices by secondary solution. An 

 analogous feature characterizes the pits on aluminous amphibole, as will 

 be noted further on. Again, the corner at C, while not definite enough 

 to allow of exact location as a point, can be with low powers so fixed 

 with reference to A and the trace of the cleavage as to orient immediately 

 the whole figure. Measurements were made of the angle CAH, within 

 about 2° of accuracy in each case, as follows : — 



Specimen. Angle CAH. 



H. 5 12°. 



H. 10 13°-16°. 



V. 12 13°. 



V. 14 10°. 



V. 16 12°-14°. 



V. 17 12i° 



P. 19 130-15°. 



V. 20 i . 13°. 



V. 22 13°. 



This angle is then seen to vary only slightly, and it has always the same 

 sign with respect to the trace of the cleavage. We may thus state the 

 orientation of the pit by means of its longer axis -4 C; — the point 

 of the " claw " is directed towards the positive hemij^yramids. Further- 

 more, the asymmetry of the figure expressed in the contrast of the 

 curved side ADC diad the side AB gives the orientation at a moment's 

 ijlance. 



* Cf. Klocke, Zeit. fiir Kryst., 1877-78, Bd. II. p. 144. 



