238 TJie America)} Geolos^ist. October, 1900 
Fouque 'has made very accurate measurements of the re- 
fractive indices of a few speciments of "labrador-bytownite" 
from the volcanic rocks. His results are: 
Composition 
SiO.. AloOa CaO Na^O K^o" Sp. Gr. Locality n^ «m ^/p 
.")3.8 30.1 11.4 5.1 .1 2.708 Vellas,* He St. Jorge 1.5677 1.5628 1.55! 
53.4 29.4 12.5 4.8 .2 2.710 Oapello, Fayal 1.5689 1.5639 1.5G: 
54.5 29.8 11.4 4.5 .3 2.G98 Besseyre, Hte Loire 1.5637 1. 50 
•')3.7 29.6 11.8 4.9 2.703 Theoretical composition of AbgAn^. 
53.38 29.70 11.90 4.30 .56 2.701 Carlton Peak, Minnt 1.5705 1.5660 1.56 
It will be noted that the indices of the American labradorite 
are slightly higher than those determined by Fouque. 
Birefringence. The measured indices of refraction indicate 
at once the birefringence, but results much more accurate are 
readily obtained by the direct measure of the birefringence. 
This was accom.plished by means of two sections perpendicu- 
lar to 7ig and n^^. The apparatus used was the ''■coinparateiir' 
of Michel Levy, and it is believed that the limit of error does 
not exceed .0002; the results obtained were 
Direct measure Measure of the indices 
»g- n^ = .0046 .0045 
n^ - Hp= .0036 .0034 
Wg -??p= .0082 .0089 
Fouque made no direct measures of the birefringence of 
the feldspars, but the following values are derived from his 
measvires of the indices of refraction by the method of the 
prism: 
Locality Composition n^ - ?im «m - «p »ig - «p 
Besseyre, Haute Loire Ab3An4 .0030 
Capello, Fayal Ab3An4 . 0050 . 0028 . 0078 
He St. Jorge, Azores Ab3An4 .0049 .0031 ..0080 
Carlton peak, Minn AbsAn, .0046 .0036 .0082 
It will be seen that the difference between the partial bire- 
fringences is much less (.0010 instead of .0018 and .0022) in 
the case of the Carlton peak labradorite; this corresponds with 
the larger optic axial angle in the latter. Further the maxi- 
*Contains also traces of FcjOa, and .1 of MgO. 
tContains also traces of MgO, and .21 of Fe^Os. 
