BEAVER BAY GROUP BELOW GRAND MARAIS. Ba | 
arborescent and netted secondary quartz. Calcite seams permeate much 
of the rock, and, in many places, the weathering has so affected it that a 
slight blow of the hammer on the face of the cliff will bring down showers 
of angular fragments. This peculiar result of weathering is very charac- 
teristic of the Lake Superior red felsites. 
The large exposures of red felsite on the Devil’s Track River have 
already been alluded to. From the mouth of the river nearly to the north 
line of See. 3, T. 61, R. 1 E., the exposures are almost continuous and the 
cliffs occasionally rise to a height of 150 feet. There is often an appear- 
ance of lamination, and across these markings a strong columnar struc- 
ture is frequently seen. The thin section shows the usual non-polarizing 
base, with ferrite particles and net-worked secondary quartz. 
Below the Devil’s Track are long beaches of felsite shingle. In See. 9, 
T. 61, R. 2 E., typical brown ashbed-diabase and diabase-porphyrite form 
the shore, and at one point show a capping mass, five feet thick, of curi- 
ously intermingled sandstone and amygdaloid. From this point to the 
mouth of the Brulé River red shingle beaches are interrupted occasionally 
by exposures of brown and red diabase-porphyrite. Many ledges show no 
macroscopically visible porphyritic ingredients, but are porphyrites because 
of their content of non-polarizing base; others show numerous large por- 
phyritic oligoclases and augites. In the groundmass in some slices more 
or less secondary ramifying quartz is seen, when we have a transition to 
the rock recognized in the pebbles of the South Shore conglomerates as a 
non-quartziferous porphyry. 
Below the Brulé, coarse olivine-gabbro forms the coast for a number 
of miles, lying in flat, often cross-columnar flows. Just below the mouth 
of the Brulé this rock is peculiar, on account of its great richness in brassy- 
lustered diallage and its very large olivines, which are sometimes one-fourth 
to one-third of an inch across, and always altered to a black, resinous-looking 
substance with a concentric scaly structure (hyalosiderite). Below the 
large bay in the northeast part of 'T. 62, R. 4 E., these coarse rocks are 
interrupted by beaches with detached ledges of dense, brown diabase-por- 
phyrite. One exposure, near the west line of See. 12, T. 62, R.4 E., shows a 
red crystalline rock, which macroscopically presents a mass of red feldspar 
21L 8 
