DALY. — ETCH-FIGURES ON AMPHIBOLES. 395 



faces and the edges are curved, AC more than AD and DC more than 

 AC. AB is a part oi AG which is practically straight, but of variable 

 length when coinjiared to the pit as a whole. A, B, 6', and D being the 

 principal angular points of the figure on the outside, i. e. marking the 

 more or less clearly defined corners formed by the meeting of pairs 

 of figure-faces and the prism-face etched, A', B', and D' are corre- 

 sponding corners at the bottom of the pit. Their position changes with 

 the maturing of the figure, but they are always analogous to A, B, 

 and D, because the bottom figure-face remains parallel to (110). To get 

 some idea of the relative dimensions of the j^its we shall define the 

 " length " of a figure as the distance, ^l H, from A to the foot of the 

 perpendicular running from C to the trace of the cleavage passing 

 through^. The "breadth" is, in like manner, the distance from this 

 trace to the line tangent to the curved side D C and parallel to the 

 cleavage AH. The maximum* length observed was about one tenth of 

 a millimeter and the breadth never far from one third of the length. 

 The angle D' A' B' gives an indication of the bluutness of the figure, 

 and is selected for measurement rather than D A B on account of its 

 greater sharpness, and hence the greater accuracy in measurement. It 

 is strikingly constant at from 72° to 73°. The angle C A His of course 

 variable with the elongation of the figure, but preserves a north by east 

 trend in all cases. Its value is usually from 13° to 15°. 



The angle BAH also helps to orient the figure, and displays an 

 interesting relation to the deepening of the pit. Tiie shallow initial pits 

 of V. 46 characteristically had a large value for this angle (== 5°) ; as 

 they matured, it passed through intermediate values until parallelism 

 oi AB with the trace of the cleavage was reached; there the swivelling 

 o( AB seems to have been arrested in even much older figures, and thus 

 the angle BAIT never was observed to change its sign. A cause for 

 this swinging of ^ i? is problematical, but it looks as if Becke's* prin- 

 ciple of differential solution might be used in explanation. In the corner 

 A, there is likely to be more rapid saturation with the jjroducts of solu- 

 tion than along the medial part of the figure-face A C, since these are not 

 so readily whisked away in convection currents acting on the constricted 

 parts of the pit as in those affecting the more open region about the 

 point B, for example. Thus, the middle part of AC will suffer in a 

 unit of time the attack of a greater number of chemically active ions 

 than will the corners, and yield faster to solution accordingly. Such 



* Min. und petrog. Mittheil., 1885, Bd. VII. p. 240. 



