Gabbroid Rocks of Mimiesota. — Winchell. 279 
mesotype,or natrolite,scolecite, and stilbite; it differs from the 
first in its birefringence which is very much weaker, the very 
small optic angle, the elongation of the fibers, always positive^ 
and the maximum extinction angle in the zone of the vertical 
axis (0° in thomsonite); from the second in the very small 
optic angle, the weaker birefringence, the maximum extinc- 
tion angle in the vertical zone (0° in mesotype), and in the 
presence of calcium in notable amount. 
Pseudomesolite is distinguished from scolecite* by the sign 
of the acute bisectrix, by the weaker birefringence, by the 
positive elongation, by the small optic angle, and by the rarity 
(or complete absence) of twinning. 
Pseudomesolite differs from fibrous stilbite in its small op- 
tic angle about the positive bisectrix, in the sign of the elong- 
ation, in the magnitude of the maximum extinction angle in 
sections parallel to the elongation, in the slightly weaker bire- 
fringence, and in the rarity or absence of twinning. 
Magnetite is not rare as a secondary mineral. 
Hematite is especially common at Carlton peak, appearing 
in nearly all the original minerals as soon as they begin to 
alter. It occurs in the form of very minute particles and 
lamellae, usually along cleavage or fracture lines. In thin sec- 
tion hematite is transparent only when in very thin scales, 
whose color is blood red; the color by reflected light is always 
red with metallic luster. A weak pleochroism can sometimes 
be detected with: 
ng •=blood red. 
wp =slightly paler red. 
The absorption is therefore: ng > //p 
In thin section it is nearly impossible to distinguish hema- 
tite from gothite, which has all the characters above noted. 
To make this distinction a part of the rock highly altered was 
heated over the Bunsen burner for half an hour, and a thin 
section then made. This section showed that the mineral was 
hematite, since the heat had in no way affected it. The hema- 
tite has not been observed to alter except perhaps to magne- 
tite. 
Pyritc and Limonite. The alteration of the magnetite is not 
*Scolecite may contain considerable sodium. 
