DALY. — ETCH-FIGURES ON AMPHIBOLES. 423 



by an angle of 180°. The reasons for altering the old orientation have 

 been so well expressed by Williams that they here need no more than 

 mere mention. (1) The base of pyroxene is an important plane on 

 account of the mineral's well known habit of twinning parallel to that 

 face, and, secondly, on account of the (probably resulting) planes of 

 parting that are so often developed parallel to (001). Amphibole shows 

 both phenomena with reference to the unit dome of the old orientation. 



(2) Parallel intergrowths of the two minerals are much more intelligi- 

 ble if this unit dome of amphibole, sensibly parallel as it is, to the 

 base of the pyroxene, be really regarded as the base of the amphibole. 



(3) There can be no doubt that the optical and other properties of the 

 two groups can be more easily compared in the new orientation. 



Now, when we remember that the pits on (100) are in both families 

 boat-shaped figures with the bow of the boat headed in opposite direc- 

 tions if amphibole be placed in the old orientation, in the same direction 

 if in the new ; that the two kinds of joits respectively characteristic of 

 the clinopinacoid on diopside and actinolite are practically identical in 

 arrangement in the new position advocated ; that the cohesional relations 

 of the aluminous amphiboles as regards (010) witness to the same close 

 relationship; and that the analogy of the figures on the two sorts of 

 prismatic cleavage is so well brought out in the greater bluntness of the 

 upper end in each case ; — it is undeniable that the physico-chemical facts 

 of corrosion with the acid render it highly expedient to follow Dana 

 in his " System," and Lacroix in " La Mineralogie de la France," in 

 reversing, for purposes of systematic comparison, the orientation so 

 recently, and with so little reason, advocated by Hintze.* 



Optical Orientation of an Amphibole Crystal or Cleavage 

 Plate by Means of Etch-Figures. 



I cannot subscribe to the opinion of Pelikan,t that the etch-pits on 

 the orthopinacoid and on the clinopinacoid of diopside are valueless for 

 purposes of optical orientation. Whether it be because of different 

 methods of procedure or not, yet I have always found a minimum of 

 difficulty in applying Wiilfing's directions for the employment of etch- 

 figures to this end. The same facility of use characterizes the corre- 

 sponding figures on amphiboles. For their actual application, as well 

 as for that of the pits on (110), the reader is referred to the type 



* Handbucli fiir Mineralogie, p. 1186. 

 t Op. cit., 1896, p. 12. 



