640 General Notes. 
MINERALOGY AND PETROGRAPHY}! 
o : tion of from 44°-50°, and magnetite. In the augite th 
, pleochroism is a + b > c. By the action of the volcanic 
Ta i reddish-yellow greenish í artzife r- 
gases the basalt is almost completely changed into a qu id 
ous aggregate in which are occasional grains of augite and plag 
shows it to contain from fifty-six to fifty-nine per cent of silica, 
while the altered variety contains as high as ninety per 
_ * Edited by W. S. BavLey, Johns Hopkins University, Baltimore, Md. 
Bulletin de l’ Acad. Roy. de Belgique, 111, 2, p. 95- 
Tb., Mil, 2, p. 10s. 
Tib m1, 2, p. I2. 
SIb., 1, 2, p. 156, 
