212 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
the white bands being merely irregular rows of light blotches. The indi- 
vidual blotches do not average more than a small fraction of an inch in 
length, the red material closing together between them. Under the micro- 
scope both light and dark portions show little sheaf-like aggregations of 
orthoclase, which mineral in the whiter portions is often larger than in the 
red. A good deal of the usual secondary quartz and ferrite are present. 
The porphyritic orthoclases are much decomposed and reddened, and are 
not very abundant. 
Another banded rock (1263) is shown in one of the numerous north- 
easterly trending ledges of the southern part of T. 51, R. 43 W., near the 
southeast corner of section 32. Thisis a dark-red felsite without porphyritic 
ingredients, looking much like a dark-red quartzite, with close and quite 
regular bands produced by variation in depth of color. Under the mi- 
croscope the thin section shows but little coloring matter. The lighter bands 
are seen to be composed of rather coarser particles than the others, while 
the banding is further emphasized by fine lines of thickly crowded ferrite 
particles. The base appears to be composed of individualized orthoclase 
and quartz, and some little quartz that appears as if secondary, but there 
is none of the irregular mossy or radiating or concentric structure produced 
by secondary quartz, such as noticed in other sections, the base presenting a 
remarkably homogeneous, finely crystalline appearance The numerous 
samples brought from other ledges within the porphyry area of the Poreu- 
pines do not indicate the existence of any other phase of the porphyry than 
those described above. 
The limits of the Porcupine porphyry area are pretty well determined 
by exposures, more especially on the north and northwest, where the ex- 
posures of the different rocks often lie quite close together. Thus in sections 
30 and 31, T. 49, R. 44 W., the Presqu’ Isle River exposes diabases and 
amygdaloids of the ordinary type, and the latter rock is seen again in the 
southwest of section 19. In the northeast of section 19 and the northwest of 
section 20 are large exposures of red felsite and felsitic porphyry, while in the 
middle of the west side of section 18 are seen the amygdaloids and diabase 
belonging to the belt north of the porphyry, the north and south limits of 
which on the Presquw’ Isle are thus indicated. Again, at the rapids of Little 
ies. 
