cae 
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ROCKS OF PORTAGE BAY ISLAND. 297 
structure, the rock coming out in thin slabs. The intervening beaches are 
presumably occupied by amygdaloids. This compact rock resembles, both 
externally and beneath the microscope, some of the conchoidally fracturing 
layers of the Agate Bay Group (as for instance that above described as 
occurring on the shore near Silver Creek, Sec. 22, T. 53, R. 10 E.), and 
also some of the layers of the beds of the Duluth and Lester River groups. 
Below these brown rocks no exposures were found for about a mile, 
when, four miles above Portage Bay Island, dark-gray, fine-grained diabase 
pseud-amygdaloids alternate with amygdaloids in which the matrix is dark 
gray and the amygdules rather sparse and large and chiefly of stilbite and 
calcite. Similar rocks are seen again after a long beach, at about three miles 
above Portage Bay Island. After this another long beach intervenes, 
beyond which there is first a low, irregular exposure of a coarse-grained 
black gabbro, which is possibly a dike, and then, just at the west point of 
Grand Portage Bay, dark-gray, medium-grained diabases with amygdaloids 
like those just described. These gray diabases and amygdaloids are not 
much like anything else seen on the Minnesota coast. 
On Portage Bay Island still lower rocks are visible. On the northeast 
corner of the island are the uppermost beds of the Animikie Group, seen 
near the water’s edge. Overlying these beds, and forming the mass of the 
island, is a considerable thickness of an aphanitic black rock, which under 
the microscope appears to be made up chiefly of augite in aggregations of 
rounded grains and magnetite particles. The plagioclases are subordinated 
to the feldspars in quantity, the proportion varying in different sections. 
Fic. 16.—Generalized section of Portage Bay Island, Minnesota coast. 
Some sections show more or less of an isotropic material penetrated by 
minute plagioclases, as in the matrices of many of the amygdaloids. Other 
sections show numerous chlorite pseud-amygdules. In one place near the 
