DALY. VARIETY OP HORNBLENDE. 435 



latter case, the darker areas of the section generally occur at the ends of 

 the crystal, representing a late stage of growth, and are, with the rest 

 of the crystal, commonly covered with a thin mantle of still a third kind 

 of substance considerably lighter in tone than either of the other two. 



Accurate sections cut in the appropriate directions by M. Werlein of 

 Paris enabled me to determine the chief optical properties of the horn- 

 blende. The optical plane is parallel to the plane of symmetry. The 

 axis of least elasticity lies in the obtuse angle (i (Tschermak's orienta- 

 tion), making an angle of 15° 9' with the vertical axis using yellow light 

 or 15° 5' using white light. The mineral is negative. In oil with an 

 index of refraction of 1.609, I found the optical angle (2 H) to be 

 53° 24'; in another oil with an index of refraction of 1.5011, I deter- 

 mined 2 H to be 57° 24'. The hyperbolas were not well defined, and, 

 on account of strong absorption, the readings had to be made in the 

 brightest white light procurable. 



Owing to the extreme ease with which the mineral cleaves, it was 

 found impossible to cut oriented prisms for the purpose of finding the in- 

 dices of refraction ; nor was any other method feasible under the cir- 

 cumstances. The true optical angle cannot then be found from 2 H, 

 since the mean index of refraction is not known. It may, however, be 

 considered that this index lies withm the limits of 1.622 (tremolite) and 

 1.725 (hornblende, a high value). The first reading for 2 H (53° 24') 

 would give for b = 1.622, 2 V = 52° 56', and for 6 = 1.725, 2 V = 

 49' 42'. The second reading (57° 24') would give for the same values 

 of b, 2 V = 52° 46' and 49° 24'. The closeness of the agreement in the 

 respective calculated values of 2 V is rather fortuitous. The optical angle 

 for this section is, then, within a degree or so of 50°. 



The double refraction seems to be low. The dispersion is weak 



(p<^)- 



On (110) the extinction varies with the zones, increasing with the 

 depth of tint. ' One dark zone gave in white light an average reading of 

 20° 53' ; other lighter zones afforded extinction angles as low as 17°. 

 The total range, so far as observed, lies between 22° 30' and 17°. This 

 can only mean that the optical angle for the different zones varies and 

 must have values between 42° and 60°. (See Figure 3.) 



The pleochroism is very strong in characteristic colors : - 



a = light brownish green. 

 b =1 dark yellow green. 

 t = dark blue green. 



b >c>a 



