68 



NEW YORK STATE MUSEUM 



The specific gravity agrees closely with that of augite giving 

 for an average of several determinations on different crystals, 



The extinction angle, as determined on a section cut parallel of 

 oio gave c C=54°. Thin sections show marked pleochroism as 

 follows : 



q — olive-green, ft = greenish yellow, a = bluish green. 



Amphibole 



Amphibole which is by far the most prominent mineral asso- 

 ciated with the magnetite of Lyon Mountain, occurs in two varie- 

 ties. Of these, a black hornblende occurring in immense crystals 

 represents a phase of the same generation as the first generation of 

 quartz previously described. The crystals of hornblende, which 

 show the effects of partial resolution, are more or less corroded 

 exhibiting rough and etched surfaces particularly on planes of the 

 clinodomes. In some instances dendritic deposits of secondary 

 stilbite were noted on the planes of the prismatic zone, a charac- 

 teristic example of which is shown in plate 9. The following 

 forms were observed: b (010), m (no), e (130), t (ioi), p (101), 

 r (on), i (031) and 0(121). The habit is shown in figures 17 a- 17b. 

 The reflections obtained from the faces were in most instances 

 poor but the values of the measured angles corresponded with 

 theory to a sufficient extent to admit of the identification of forms 

 of such ordinary occurrence. 



SUMMARY OF MEASURED AND CALCULATED ANGLES 



Angle 



Measured 



Calculated 



Angle 



Meas 



ured 



Calculated 





/ 



/ 









/ 



/ 



b:e 



32 20 



32 II 



m' :z 



68 



46 



69 29 



b:m 



62 7* 



62 5 



m r :r 



96 



17 



96 10J 



b:r 



74 15 



74 14 



z:z' 



58 



5o 



59 9§ 



b:i 



50 19 



49 44 



p:r 



34 



22 



34 25 









p:t 



55 



O 



55 4 



Cleavage parallel to m is highly perfect, producing cleavage faces 

 of remarkable brilliancy. The specific gravity is quite high 



