W. E. Ford — Optical Study of the Amphiboles. 187 



percentages of the alkalies. It is especially distinguished by 

 the unusual amount of fluorine that it contains (2*76 per cent). 

 Another factor to be considered is the 1'2 per cent of Ti0 2 

 which is present. The effect of an increase in the percentages 

 of fluorine and the alkalies is generally thought to lower the 

 indices of refraction and that of an increase of Ti0 2 to be 

 opposite in character. With all these constituents present in 

 unusual amounts it is difficult to come to any reasonable con- 

 clusion concerning the correlation of the chemical and optical 

 properties of this amphibole. The axial angle, diverging as it 

 does so markedly from the usual value, shows clearly that this 

 Grenville mineral is abnormal in character. 



Analysis 13 is of the hornblende known as pargasite from 

 Pargas, Finland. Its optical characters as given bv Kreutz 

 arenas follows: a = 1-6158, /3 = 1-6205, 7 == 1-6353. Mean 

 index =1-6255. y-a - "0195. 2Y = 59° 30'. c^c=27°13'. 

 As Kreutz has pointed out, this mineral is closely analogous to 

 the hornblende from Grenville described above. Like it, it is 

 characterized by a low amount of silica, high percentages of 

 alumina and alkalies and by the presence of a notable amount 

 of fluorine (1*82 per cent). The extinction angle and the 

 axial angle are also closely similar. Like the Grenville mineral 

 it cannot be considered as normal in character. 



Analysis No. 11 is of a hornblende from Chester, Massachu- 

 setts, described by Duparc and Pearce. Its optical properties 

 are given as follows : a = 1-6598, /3 = 1-6729, 7 = 1-6798. 

 Mean index = 1-6698. y-a = -0200. 2V = 75° 44' (calculated 

 by present writer). CaC = 11° 10'. This analysis agrees rea- 

 sonably well with what might be predicted for an amphibole 

 having its mean index of refraction. The amounts of silica 

 and magnesia agree closely with the corresponding percentages 

 to be derived from figures 1 and 7. The amount of sesqui- 

 oxides and the total iron are somewhat higher than would be 

 expected. 



Analysis No. 15 is of a variety of amphibole called soretite, 

 coming from Koswinsky in the northern Urals and described 

 by Duparc and Pearce. Its optical character is as follows : 

 a = 1-6627, £ = 1-6765, 7 = 1-6856. Mean index = 1-6741. 

 7 -a = -0228. 2Y = 82° 30' (observed). Csd = 17°. Pleochro- 

 ism, a = pale greenish yellow ; b = green, c = deep green. The 

 analysis agrees quite closely with the theoretical one to be 

 derived from consideration of the figures 1 to 9. The amounts 

 of silica, alumina and magnesia are within less than two per 

 cent of what would be expected. The percentage of ferric 

 oxide is high and that of ferrous oxide correspondingly low t 

 but the amount of total iron present is almost exactly that 

 which would have been predicted from the study of figure 6. 



