1896.] Mineralogy and Orystallography. 811 
The plane of the optical axes is the base, and the optical angle is 
85° 38’ for lithium, 85° 45’ for sodium, and 85° 56’ for the thallium 
light. 
The results of analysis are: 
I. I. 
SiO, 42.65 42.39 
FeO 1.35 3.12 
MgO 56.57 55.09 
CaO 29 
Al,O, 23 
100.86 100.83 
Sp. G. 3.223 3.245 
The ratios RO: SiO, are 2.018:1 and 2.01:1 respectively, after de- 
duction for probable impurities. 
Fayalite and the Chrysolite-Fayalite Group.—Penfield and 
Forbes found the fayalite from Rockport, Mass., suitable for optical 
and other investigations. The mineral was found in the shape of a 
lenticular shell in massive hornblende-biotite granite. The color is a 
dark resinous green, though the light transmitted by the thin edges is 
yellowish. The purified powder has a specific gravity of 4.318 (aver- 
age of 3 determinations). The average of the two analyses is: 
SiO, 30.08 
FeO 68.12 
MnO 72 
H,O 80 
99.80 
The cleavages are 001 and 010, and the reported occurrence of a 
cleavage 100 is considered a mistake. 
The plane of the optical axes is the base, and the double refraction is 
negative. For sodium light. =» 1.8236, = 1.8642, y= 1.8736, 
giving y-= =.050. The macro-axis is the acute bisectrix, Vy = 25° 
18’, 
A specimen of hortonolite from Monroe, N. Y., was also investigated. 
The table given below exhibits at a glance the effect of the iron on the 
optical characters of the chrysolite-fayalite group : 
5 Am. Jour. Sci., CLI, pp. 129=135, Feb., 1896. 
