Gabbroid Rocks of Minnesota. — Winchell. 165 
integrating easily. This bedded structure is probably not due 
to pressure or to any process of differentiation but is ascribed 
by Winchell * to the original sedimentary condition of the 
formation. 
The color of the rock is in general a dark grayish green 
varying to dark green, as the ferromagnesian minerals become 
more abundant. Fresh fractures show the bright lustrous 
cleavages of the feldspar, as well as the much darker cleavages 
of the pyroxene. The common albite twinning striations are 
visible nearly always on the feldspar, which is often more 
abundant than the pyroxene. The plagioclase anhedra are 
usually larger and perfectly fresh and glassy. The augite is 
jet black in mass, but thin splinters are greenish-black. It 
usually crystallizes without any definite form, but is occasion- 
ally triangular or wedge-shaped. Magnetite is easily detected 
also by its bright metallic reflections of blue black color; it is 
often abundant. Biotite is often conspicuous by reason of the 
perfection of its cleavage, giving the brightest reflections of 
a brown to black color, from faces absolutely uniform; but 
microscopic study shows that the biotite is usually, if not al- 
ways, a secondary reaction product formed from the feldspar 
and the ferriferous minerals. Olivine can be detected by the 
olive-green to yellow color of the rather small crystal grains. 
As elsewhere noticed it is only rarely entirely lacking, though 
it is usually not abundant. Pyrite sometimes occurs very 
sparingly. When decomposed the feldspar becomes white and 
opaque, apparently an aggregate of minute grains. At the 
same time the olivine loses the lustre and transparency of its 
perfect state, and changes to an earthy mass of minerals be- 
longing to the group of antigorite; they are usually dark 
green. The augite may also alter, first to biotite and then to 
chlorite. 
The texture of the normal gabbro is always granitic, 
usually coarse to medium grained. (See plate VIII, figure 1.) 
It is only when it solidified near the surface, usually in the 
form of intrusive sheets, which occur along the northern edge 
of the gabbro mass, and more commonly in the northeastern 
part, west from Pigeon point, that the rock passes into a dia- 
*N. H. Winchell: Final Report: Geol. Nat. Hist. Surv. Minn., 
Vol. IV and V, 1899 and 1900. 
