Vale THE JOCKNALVOD, GHOLOGY. 
sion. ‘The Pewabic quartzite at the bottom of the Animikie decreases 
in thickness as Birch Lake is approached from the west, and in the 
vicinity of Iron Lake disappears entirely. From this locality eastward 
the iron-bearing rock rests upon the granite. It is concluded that the 
Pewabic quartzite between Birch Lake and Gunflint Lake belongs to 
the middle iron-bearing member of the Animikie. 
Ines o2Nq and orsN. Ns ro) We andi rn We soccurse am ieavily 
bedded olivine gabbro. In going from the northern and southern 
limits of the gabbro toward the center of the area it is noticeable that 
the ferro-magnesian minerals decrease and the feldspar increases in 
proportion, until in the center of the mass occur numerous knobs and 
areas, a mile or more in extent, composed of plagioclase rock or anor- 
thosite, which are regarded as segregations. In the center of the mass 
the rock has greater coarseness of texture, and also more of a stratified 
appearance, arising from the arrangement of the constituent minerals 
in bands. The mineral and chemical composition of the various parts 
of the formation correspond to the known rules which govern the 
cooling of liquid magmas, and the whole is regarded as a batholitic 
intrusion rather than a surface flow. 
Red rocks, comprising augite-syenite, quartz-porphyry, felsite, etc., 
occur in the vicinity of Greenwood Lake, and were followed to the 
shores of Lake Superior, making together one prominent group of 
rocks. 
The dark gabbros of Irving, the diabases and the amygdaloids are 
placed in another group, called the diabase group. The anorthosites 
of the coast of Lake Superior, described by Lawson as pre-Keweenawan, 
and newly discovered masses back from the coast, are found to be 
detached blocks from the great gabbro mass enclosed in and underlain 
by the black gabbro, as previously held by Irving and Winchell. The 
latter rock is considered as the effusive equivalent of the great basal 
gabbro. After the aggregations of feldspar had separated from the 
gabbro magma, and were floating around in it, they were ejected in 
portions of the unsolidified magma, and being lighter than it, floated 
near the surface, and are found only near the top of the first outburst of 
lava or black gabbro. Later, when the rock was somewhat eroded, 
the feldspar knobs projected above the surrounding rocks, and later 
were covered by the flows of the red rock group. Therefore, the con- 
clusion of Lawson that the anorthosite forms a pre-Keweenawan 
terrane, is rendered valueless. 
