fih* AmphiboL and Pyroxem groups. 



The peculiar minerals which have been described in the pre- 

 ceding pages are of interest from several different standpoints. 

 In the first place, as to the identity of the varieties observed, 

 ir seems clear, either, that they represent a group of known, 

 allied forms, occurring here under quite new and remarkable 

 conditions, or, they constitute an equally interesting group of 

 new varieties. The former alternative Beems the more prob 

 able, and the blue amphiboleis to be compared with riebeckite } 

 the new Bpecies recently described by Saner,- while the green 

 pyroxene is apparently csgirite, or the closely related form 

 acmite. It is also suggested that the dark brown amphibole, 

 rarely appearing with the blue, may he barkevicite. 



The blue amphibole agrees very closely with glaucophane in 

 pleochroism hut the colors of a and t are interchanged, cor- 

 responding to their different positions. With rieheckite the 

 agreement in optical characters is more nearly complete. The 

 angle 6 : a in the blue amphibole is 13°-15° in front of 6 (refer- 

 red to the usual orientation of the group), while for rieheckitef 

 c:a=5°-7°, but neither Saner nor Rosenbusch state whether 

 tin- axis a is in front of 6 or not. From the considerations 

 presented below it is probable that it is inclined in the same 

 direction as in the blue amphibole here described, and in that 

 ■ the difference in orientation is but 7°-10°. Otherwise the 

 minerals differ in intensity of color, pleochroism and absorption.} 



In his " Hiilf stabellen " Rosenbusch giyes the optical orien- 

 tation of arfvedsonite, " 6 : c(?)=14° vorn." As b=b the query 

 indicates that it may be the axis a of elasticity which is near <?, 

 and if this were true, the orientation of the blue amphibole 

 and of arfvedsonite would be nearly identical. This orien- 

 tation for arf yedsonite seems the natural one in yiew of the 

 relationships to be presented.^ 



In the dike rock there are certain individuals of dark blue 

 color in which absorption is so strong that they seem black 

 when b or the axis near 6 is parallel to the principal section of 

 the Xicol. This dark material occurs in several places as an 

 addition to actinolite, very much like that of the chestnut- 

 brown mineral in fig. 5. but it is also found associated with the 

 lighter blue amphibole in the same crystal or in separate but 

 adjacent ones. This extremely dark variety would seem to 



ve fully with rieheckite. and its presence in company with 



♦Zeitschrift der dentsch. geol. Gesellschaft. xl. 138, 1888. 



f Sauer, loo. cit. Rosenbusch, Hulfetabellen but niikr. Mineralhi'stinnniing, 

 1888. 



% While reading the proof of this article the writer's attention was drawn to 

 the observations of A. Lacroix on the optical propeitk-s of crocidolite (BulL E 

 Fr. de Mineralogic. xiii. p. 10, Jan., 1890), showing that mineral to be very closely 

 related t<> rieheckite and to the blue amphibole h< Wed. 



^ Lacroix has Btated (Comptes Rendus, etc.. cix, ."'•:•) that tin- amphibole from 

 near Pike's Peak, hitherto cousidered to be arrVedsonite on the ground of Kf>i 

 analysis, has the optical orientation of rieheckito. 



