140 
BULLETIN OF THE LABORATORIES 
in the section and completely felsite. The figure shows one of the 
quartz crystals cut exactly transversely. The only other conspicuous 
ingredient is orthoclase. This occurs in crystalline form, frequently 
in baveno twins, but never unaltered. 
No. 1099b is found a short distance from the above. It is of 
lighter color than No. 1099a, and has a more distinctly conchoidal frac- 
ture. Large crystals of quartz, about an eighth of inch wide, and de- 
composing feldspar crystals of light reddish color, one-fourth of an 
inch in diameter are readily distinguished. The quartz crystals are 
regularly filled with inclusions. Besides the larger crystals of quartz, 
there are present smaller grains in large quantities, these latter of 
about the same relative size. There is no free iron oxide present in 
either specimen. Besides these two typical porphyries, there are two- 
other kinds, which on account of their weathering can not be satisfac- 
torily studied. One coarsely granular variety is light red, with whitish 
grains, (orthoclase .^), scattered throughout it. These grains can not 
be identified because of their decomposition. This rock has the char- 
acteristics of a porphyry, but greatly resembles, superficially, the con- 
glomerate. The other, occurring but a short distance from No. 1009b, 
is apparently fragmental, but the fragments are angular and irregularly 
aggregated, and in section appear like the felsitic magma of the adja- 
cent porphyry. No critical examination even of our inadequate 
material is now possible. 
In conclusion, then, the north shore of lake Superior presents 
three distinct groups of rocks with their respective intrusives, which 
may be designated as granitic, schistose and conglomeritic. The gran- 
ites are coarsely granular and contain small amounts of hornblende or 
mica and are occasionally augitic. The quartz is in large amount, 
while plagioclase is often present. These granites are found underly- 
ing the schists in such a way as to suggest that they have been intrud- 
ed beneath them, though of course the material must have previously 
existed. That the material was granitic is indicated by the fact that 
precisely similar granites associated with felsites constitute the pebbles 
of the basement conglomerates in the schists. The origin of this old- 
er granite is unknown, but it would seem that it occurred in the same 
way as the existing exposures beneath sedimentary or other masses 
which have been used in the formation of the schists. 
The eruptives found. perforating the granite are all diabases. It 
is not denied that there may be felsite dykes, but none were observed. 
