420 PROCEEDINGS OF THE AMERICAN ACADEMY. 



astonishing to observe the very close parallelism which even a cursory 

 study established between the etching properties of the two mineral 

 genera. This is one part of the present investigation which the author 

 regrets to leave in an especially incomplete stage. Thus, it would be 

 of moment to produce etch-figures on that prism of pyroxene (210), 

 which is nearly equivalent to (110) of amphibole, and compare them with 

 those of the last-mentioned face. This has been left undone for lack of 

 material. But even the facts in hand are wonderfully accordant. To 

 simplify matters, I shall enumerate some of the main conclusions we have 

 reached regarding the etch-pits on amphiboles and note the comparisons 

 with pyroxene in connection with each. 



1. Actinolitic amphiboles give one class of etch-pits, aluminous am- 

 phiboles anotber, — especially evident on (010), (110), and (101). I 

 have been able to find a similar strong contrast between diopside and 

 augite in this respect: they were etched on (010) and (110). 



2. Not only is such a double cleavage of the groups possible ; there 

 are positive similarities in the respective pits on the pinacoids (010) 

 and (100) of the non-aluminous amphibole actinolite and the non- 

 aluminous pyroxene diopside, and there are positive resemblances char- 

 acterizing the pits on the same planes of augite and hornblende. The 

 paper of Pelikan,* referred to in detail further on, along with those 

 of Wltlfing t and Baumhauer J show clearly that the etching phe- 

 nomena (outline of figures, number of pits and of figure-faces, orien- 

 tation of figures, etc.) on (010) of diopside, using hydrofluoric acid, 

 are hardly distinguishable from those on actinolite. (Compare Photo- 

 graphs 12 and 13.) Pelikan § has shown, in addition, that the augites 

 from Vesuvius, Laacher See, Wolfsberg, etc., are characterized by only 

 one sort of pit on the clinopinacoid and thus are in contrast to diopside, 

 and, as we now see, are analogous to hornblende. It must be confessed 

 that no amphibole which I have yet etched has yielded anything like so 

 perfect figures on the orthopinacoid as those readily procurable on the 

 same face of Ala diopside (see Photograph 15). While the general re- 

 semblance to the pits on (100) of actinolite is certainly great, I cannot say 

 whether or not the pair of figure-faces at the lower end of the diopside 

 pit is represented in the actinolite pit. (Plate I. Fig. 27, is somewhat 

 hypothetical as to the " primary " figure-faces.) I have not been able 



* Min. u. petr. Mitth., 1896, Bd. XVI. p. 1. 

 t Die Pyroxen Familie, Heidelberg. 

 I Poggen. Annalen, Bd. CLIII. p. 75. 

 § Op. cit, p, 21. 



