Laurentian Rocks 129 
pale variety, leucoxene, surrounding Opaque masses of iron 
oxide. Thick crystals of apatite often join the preceding 
minerals. Minute zircons may be seen especially in quartz 
and the folspars. Magnetite, titanite and apatite with 
zircon are the only minerals usually idiomorphic. The large 
amount of titanium is worthy of mention, for titanite often 
equals the biotite in quantity. 
That the granite has undergone strains since solidifying 
is proved by the frequent occurrence of undulatory extinc- 
tion in quartz and felspar, as well as cataclase structure on 
the edge of the larger constituents. 
The gneiss shows in most parts the usual wavy, sweep- 
ing curves of foliation including lenses of felspar and larger 
enclosures of quartzite, and is much more varied in char- 
acter than the granite. It commonly contains a larger 
proportion of biotite than the granite, and hence is darker 
in color, though one variety from the mainland is nearly 
white and formed of thin layers of quartz and felspar almost 
free from mica. In grain the rock varies from coarse peg- 
matitic bands with felspars three inches across to very fine 
grained portions, almost halleflintas. 
A curious structure, probably resulting from a shearing 
motion, may be seen crossing both granite and gneiss, vein- 
like bands an inch or two wide, in which the basic minerals 
have almost disappeared, and quartz and felspar have been 
rolled into fine parallel plates, from which curves sweep off 
into the unaltered rock around, the curve on one side re- 
versing the direction of that on the other. 
Under the microscope the gneisses prove to contain all 
the minerals found in the neighboring granite except mi- 
crocline, which is almost entirely replaced by orthoclase, 
generally with microperthitic inclusions. In most cases 
biotite is plentiful, hornblende absent, quartz sparingly pre- 
sent. This is, however, not the case in varieties resembling 
halleflinta where quartz is the mineral found in largest 
amounts. Apatite occurs as needles and not as thick prisms, 
the form usual in the granite. 
