Gabbroid Rocks of Mi?inesota. — Winchell. 217 
from the coast of Labrador: or, at least, the play of colors is 
rare and indistinct. \\. is of a prevailingly gray color, varying 
from light gray to dark greenish gray. In its purest state, 
which is not uncommon, it is clear and glassy, and transparent 
even in masses four to five centimeters thick. The remarks of 
Prof. A. C. Lawson, on the structure as seen in the field, dur- 
ing the summer of 1893, follow:* 
"Another remarkable characteristic of the rock as a forma- 
tion is its massiveness and lack of structural planes of any 
kind, except under some abnormal circumstances. Even joint- 
age may be said to be entirely absent. Occasionally the for- 
mation is traversed by one or more fissures locally, but these 
follow no law of direction and cannot be regarded as true 
jointage. There are few of the massive rocks of the lake Su- 
perior region so free from structural planes. There is no iiow 
or gneissic structure, and the rock seems nowhere to have 
been subjected to forces which would tend to deform the mass, 
render it schistose, or in any way induce the development of 
secondary structures of a mechanical nature." 
At the first glance the rock often presents the appearance 
of being composed of a single mineral, a feldspar with fine 
cleavage often showing very distinctly the albite twinning 
striations. But a closer view will disclose the presence of 
small grains of dull greenish black augite, distributed rather 
evenly through the rock; usually filling the interstices be- 
tween the feldspar crystals,and but rarely possessing any crys- 
tal faces. Besides these two minerals a third is nearly always 
present, iii the shape of a bright, black mineral with metallic 
blue-black reflections. It is magnetite. 
When the rock is weathered and decomposition has set in, 
it is common to see hematite appear and the rock take on a 
reddish tinge, but at the same time the feldspar alters along 
its cleavages, which thus become quite conspicuous, and the 
division into lamellae gives a dull white appearance to the 
mass. This color is also due to the fact that the feldspar is al- 
tering to various white minerals and especially to zeolites. In- 
deed, whenever amygdules are found, they are filled with ra- 
dial masses of these minerals, which are often tinted very 
*A. C. Lawson: The anorthosytes of the Minnesota coast of lake 
Superior: Bull. No. 8, Geol. & Nat. Hist. Surv. Minn. 1893, p. 3. 
