Augite Soda- Granite from Minnesota. — Grant. 387 
by a distinct line. The outlines of these colorless cores are ir- 
regular and are usually not crystallographic planes. The green 
crj'stals and rims have a lower index of refraction, lower double 
refraction and a smaller extinction angle than the colorless 
variety. In a section of a zonal crystal cut parallel to the clino- 
pinacoid the colorless interior has an extinction angle, measured 
against the cleavage, of 40 degrees in acute angle B, while the ex- 
tinction angle of the green outer rim is about 25 degrees. From 
this it seems that the green crystals and rims contain more of the 
acmite molecule than the colorless ones. A very t3-pical fresh 
specimen (No. 86 G) of the porphyritic granite was powdered and 
the augite separated and analj^zed. This augite is fresh and un- 
altered and the powder, which fell at about a specific gravity of 3, 
is quite pure, as in this specimen of the rock the only minerals 
present were feldspar, quartz, augite and a few small fibers of sec- 
ondar}' hornblende. The analysis is as follows: 
SiOg AI2O3 Fe^Og FeO CaO MgO KjO Na^O H^O Total 
53.19 2.38 9.35 5.15 17.81 9.43 0.38 2.63 0.01 100.23 
Assuming that this represents an isomorphous mixture of the 
diopside, heddenbergite, acmite and fassaite molecules, and calcu- 
lating their relative proportions, we get approximately the result 
given below. In the considerable percentage of the acmite mole- 
cule this approaches in composition the augite of the moi'e alka- 
line rocks, the eleolite syenites. 
Diopside, Mg Ca SijOg 47 per cent. 
Heddenbergite, Ca Fe Si,0....... 27 " 
Acmite, Na Fe SijOg 21 " 
Fassaite, Mg Alj SiO(, 5 '' 
The augite is often seen altering to fine green hornblende nee- 
dles, and sometimes the needles have been developed all through 
the rock mass. An attempt has been made to measure the angles 
occurring on some of the larger cr^'stals of augite detached from 
the rock, but the faces gave such imperfect reflections that no re- 
liable results could be obtained. 
The original accessory minerals occur only in small amounts. 
Secondary hornblende is common and abundant in all the altered 
specimens of the granite, wliile original hornblende has been no- 
ticed in but three sections, and here it, together with the biotite, 
is as abundant as the augite. Biotite, aside from the case just 
mentioned, is sometimes seen in small flakes in the porphyritic 
