368 



W. Cross — Some secondary minerals 



the other amphibole indicates that the latter has a somewhat 

 different composition, though doubtless near that of riebeckite. 

 Considering the similarity existing between these minerals and 

 the group of soda-iron amphiboles and pyroxenes it is most 

 natural to suspect that the dark-brown amphibole represented 

 in fig. 5 may correspond to barkevicite, another form in which 

 the molecule Na 2 Fe 2 Si 4 12 is known to occur predominantly, 

 according to the latest tables of Groth* and Rosenbusch.f 

 In the crystal of fig. 5 it could not be clearly determined 

 whether the axis near 6 is a or c, but the direction of extinc- 

 tion, being on the same side of the vertical axis as in the 

 actinolite and hornblende, indicates that it is c. 



The green pyroxene occurring in the conglomerate resembles 

 aegirite in color, pleochroism, absorption, and optical orienta- 

 tion. It differs from it as the blue amphibole differs from 

 riebeckite, in the intensity of its pleochroism and absorption, 

 while it has a purer green color than is usual for aegirite. 

 In the characteristic development of steep terminal planes this 

 mineral contrasts strongly with the habit of aegirite of other 

 occurrences, but its development is just that observed in acmite, 

 a variety now commonly united with aegirite by mineralogists. 

 Repeated efforts have been made to isolate the blue amphi- 

 bole and the green pyroxene for chemical analysis, but 

 without success. The particles of pure material are so very 

 small and are so intimately associated with brown or green 

 hornblende or with the unknown yellowish mineral, that sep- 

 arations by specific gravity could not be effected. A few 

 grains treated with hydrofluosilicic acid yielded microscopic 

 crystals of soda and iron salts in characteristic forms. 



In studying such a number of associated amphiboles and 

 pyroxenes the writer's attention was naturally called to the 

 9. character of the variations in the position 



of the ellipsoid of elasticity within the 

 group of the amphiboles, and to a com- 

 parison of the variations presented by the 

 two groups. One of the first points ob- 

 served in studying the primary inter- 

 growths of hornblende and augite in the 

 dike rock was that the axes a and c of 

 elasticity in the two species were always 

 in similar quadrants relative to the com- 

 mon vertical axis, instead of in opposite 

 ones as would be the case were the min- 

 erals intergrown with the basal planes of 

 the commonly adopted orientations in- 

 clined in the same direction. Fig. 9 illus- 



* Tabellarische Uebersicht der Mineralien, 1889. 



\ Hiilfstabellen zur mikr. Mineralbestimmung in Gesteinen, 1888. 



