DALY. — ETCH-FIGURES ON AMPHIBOLES. 407 



The smaller pits (Figure 5 a) are invariably four-sided and elongated 

 parallel to the side the more oblique to the trace of the cleavage. So 

 extremely minute as to be at times easily overlooked, they are, moreover, 

 inconspicuous on account of their shallowness and the consequent lack 

 of contrast with the rest of the crystal surface ; they may hence be called 

 the " light " figures to distinguish them from their darker fellows of the 

 second class to be described. The angle A^ D' H was determined at from 

 3° to 5°, the auirle B' A IX averaged 108°. A' B' and A' D' were 



On the average, A' B' : A' U is about equal to 8 : 5. 



Quite different are the pits of the other kind (Figure 5 h) ; they are 

 larger, deeper, and often more numerous. The elongation is, in this 

 case, in the sense of the vertical axis of the crystal. The four-sided 

 figures are, as in the first class, parallelograms in plan, the acute angle 

 here lying, however, in the upper left hand corner. It is this corner and 

 the opposite one that are truncated by the accessory pair of figure-faces 

 already mentioned. The angle A D II is 8° instead of o°-5° in the 

 light figures. The angle £ A D varies from 82° to 88° ; the curvature 

 of the sides prevents a close determination. The limits of variation in 

 the ratio A B : AD were observed as 1 : 1 (0.04 mm. : 0.04 mm.) and 

 about 2:3 (0.032 mm.: 0.048 mm.). 



According to the character of the figure-faces (always pyramids) these 

 larger pits can be subdivided into two sub-groups in one of which the faces 

 A B and C D are equally illuminated but darker, i. e. more steeply in- 

 clined to (010) than AD and B C, also of equal brightness, while in 

 the other group the reverse relation holds. (The figure-faces are here 

 indicated by their corresponding edges of intersection with (010).) Both 

 kinds have figure-faces belonging to the same crystallographic zones, and 

 they are probably allied as are the figure-faces described by Baumhauer 

 on apatite.* He noted — that which is evident from his photographs — 

 that, as the apatite matures (deepens) its faces become steeper and steeper 

 on (0001), (" Verschleppung " of Becke). In one case, I discovered a 

 large pit having A D steeper than A B, with the bottom occupied by a 



* Resultate der Aetzmethode, p. 48. 



