DALY. ETCH-FIGURES ON AMPHIBOLES. 



405 



other amphiboles as it has been proved to be optically. The pit is here 

 a spindle-shaped well defined figure, generally about six times, rarely only 

 four times, as long as it is broad. The sjtindle is usually ideally perfect, 

 and theu the axis can be seen to make an angle with the cleavage cracks 

 of two degrees east of north on (110), two degrees west of north on 

 (ITO). Many pits show that they are bounded by two curved figure- 

 faces of unequal steepness, and hence of unequal illumination in the 

 microscope. The narrower, darker one lies to the left on (110) ; it is 

 separated from the other by a narrow light streak that corresponds to the 

 keel of the unsymmetrical canoe. The relations are enantiomorphous on 

 (ITO). Occasionally, the spindle is blunted with what appears to be an 

 imperfectly formed third figure-face that would represent the upper figure- 

 face of the Hornblende type (Plate I. Fig. 22). The photograph does 

 not give an idea of the exceeding sharpness of these figures, at least 

 as compared with most other amphiboles ; there can be no doubt that 

 the type is a distinct one and stands alone. 



The figures show the mineral to be holohedral and centrosymmetric 

 and a cleavage plate can be easily oriented in the absence of crystallo- 

 graphic data by observing the position of the adventitious third face, or 

 the direction of the spindle-axes with respect to the obtuse angle of cleav- 

 age (110 : 110), and one, say the darker, of the two longitudinal figure- 

 faces of a pit. 



Both the symmetry and orientation of arfvedsonite are, however, better 

 made out by the use of figures resulting from exposure to molten caustic 

 soda. A cleavage piece was found after 25 

 seconds' immersion to be covered with three- 

 sided pits, as depicted in Figure 4. These 

 show once again the radical difference in 

 behavior between arfvedsonite and common 

 hornblende. (Cf. Plate I. Figs. 34 and So.) 

 It is, furthermore, an interesting case, in that 

 the directions of rapid solution are here 

 transverse to those of rapid solution by the 

 acid, and, secondly, the formation of etch- 

 figures is once again seen to be independent 

 of cleavage. Figure 4. 



Non-Aluminous Amphiboles, Etch-Figures on (010). 



The actinolite of Zillerthal was found to be typical of the whole 

 group of amphiboles not containing a sesquioxide as regards the facts 



