ROCKS OF BEAVER BAY. 307 
In the same vicinity, a narrow dike of very dense greenish-black dia- 
base zigzags about in the red rock. ‘The islets I are composed of a coarse 
anorthite-rock, whilst the larger islet H is again of very fine-grained, dark- 
gray diabase. 
Extending along the north side of the neck of the point, at the mouth 
of Beaver River, C C, for a distance of over 200 paces, as indicated on the 
accompanying sketch, is a black-tinted, banded felsite. The thin sections 
of this rock show an excessively fine granular mixture of quartz and ortho- 
clase particles, which in some sections appears to include a good deal of 
isotropic matter. Opaque black and brown particles are abundant, and here 
and there an augite crystal is seen. The banding is produced by short 
seams of lighter-colored material, which under the microscope show more 
coarsely crystalline matter. The contact of this felsite with the crystalline 
rocks of the point was not observed. 
The small island on the north side of Beaver Bay, and again the 
larger island to the northeast (Cedar Island), are made up of a quartz-por- 
phyry with an exceedingly dense and conchoidally fracturing, purple-tinted 
matrix, in which are rather thickly scattered pink feldspars, averaging an 
eighth of an inch in length, and black quartzes one-fiftieth to one-thirtieth 
of an inch across. The feldspar crystals are often very distinctly arranged 
in curving lines indicating flowage, which is also shown by the similar lines of 
lighter shading in the matrix. Under the microscope the thin section of this 
rock shows much of a substance that does not affect the polarized light at all. 
Through the unindividualized base are excessively minute particles and ar- 
borescent clusters of quartz. The blackish and pinkish particles generally 
seen in these porphyries are here much sparser and smaller than usual, and 
the rock as a whole is one of the nearest to a glassy condition of any of the 
felsitic porphyries examined. The silica content is 76.83 per cent. On 
the outer island the quartz-porphyry has a very marked columnar structure, 
and presents at the same time a strong appearance of the usual flat lake- 
ward dip. . 
At the point A of the map, the common coarse gabbro is involved with a 
fine-grained black rock, as indicated in the accompanying figure. The face 
represented is the east wall of the gorge of Beaver River, near the bridge, 
