SUBORDINATE GROUPS OF THE MINNESOTA COAST. 267 
beds of rather coarse-grained orthoclase-free gabbro are included; and 
there is a little interleaved detrital matter. Thin amygdaloids of peculiar 
character cap many of the beds of the upper two-thirds of the group, but 
the amygdaloidal character never reaches so great a development as in 
some of the succeeding groups. This group is distinctly recognizable at 
both ends of the coast, and at points in the interior, wherever its course 
has been crossed. Its thickness lessens as it is followed eastward; but 
at Duluth it is not far from 5,000 feet. 
Ill. Tue Lester River Grovr.—This is a succession of heavy, distinct 
beds of fine-grained brown rocks, largely of the ashbed type. Diabase-por- 
phyrites, some of the ordinary diabases, rare beds of coarse-grained gabbro, 
and two or three belts of granitic porphyry are also included. Amygda- 
loids are almost unknown, and no detrital material has been observed. The 
rocks of this group are known at both ends of the coast, and at intervening 
points in the interior. The thickness is about 2,600 feet. 
IV. Tue Agate Bay Group.—This is a succession of relatively very 
thin beds with very highly vesicular, stratiform amygdaloids, which must 
make up two-thirds of the thickness of the group. The prevalent non- 
amygdaloidal rock is a fine-grained, olivine-bearing diabase or melaphyr. 
Towards the base are a number of layers of diabase-porphyrite, also with 
strongly developed amygdaloids. Thin seams of reddish sandstones and 
conglomerate are also included. This group forms the coast line for a 
distance of some 35 miles below the mouth of Lester River, and has been 
traced some miles farther east by exposures in the back country, but it 
does not appear at the eastern end of the Minnesota coast, having appar- 
ently quite thinned out. This fact is in accordance with the general law 
of thinning towards the east, which is obeyed by all three groups below, 
and by the one above. The thickness of the group is about 1,500 feet. 
V. Tue Beaver Bay Grour.—This group is especially characterized 
by a predominance of black, coarse-grained, olivine-bearing gabbros in 
very heavy layers without amygdaloids, and by the great abundance and 
prominence of its included red felsitic porphyries and granite-like rocks. 
There are, however, very considerable thicknesses included of fine-grained 
ashbed-diabase’, with and without amygdaloids, while the ordinary fine- 
