96 The American Geologist. February, i905 
microHtes, only, .008 mm. in cross section, which appear 
black by transmitted light. They may be all rutile since 
this mineral is common in the biotites forming a sagenitic 
network so thick as to make them sometimes opaque. 
The section contains here and there trains of large cavi- 
ties with motionless bubbles. The whole section is evenly 
dusted with minute brown ragged scales (which seem to be 
biotite) except in a somewhat regular network of branching 
bands, which do not coincide with partings between the 
quartz grains. 
Another similar specimen of the coarse quartz-biotite 
rock is of a much deeper cobalt blue and differs very mate- 
rially in thin section from the preceding. The microlites and 
sheets of cavities are wanting and the quartz is full of the 
same minute flat ragged scales of a reddish mineral appar- 
ently biotite. Scattered irregularly in the quartz ground are 
quite large grains of deep blue spinel, often black except on 
the edges. A sim.ilar blue spinel occurs at Frobisher's bay in 
a rock consisting mostly of phlogopite and calcite which 
makes part in an Archaean limestone, and we may possibly 
assume these grains and crystals to have been derived by 
the granite from an ancient crystalline limestone. These 
grains are uniformly surrounded by a red brown biotite 
sometimes in a narrow frill of small plates, sometimes in a 
broad border of coarse plates. These always show the 
strongest indications of resorption. Large round grains of 
a black ore fill the outer portion of the mica plates and 
gather in the interior in strongly developed beaded rods or 
fusiform shapes, (often occupying the prismatic cleavage and 
the diagonal thereto, — the lines of the percussion figure) 
which become so abundant as to make the plate opaque. 
These balls extend beyond the edge of the mica plate and 
even appear apart from the mica in round areas as if the 
mica had been wholly resorbed. 
Manv beautiful blue prisms radiate from the spinel or 
lie isolated ni the mica frill. They are rhombic, have 
strong absorption, reddish violet parallel with the prism, 
rich ultramarine blue and colorless at right angles to the 
same. They seem to be dumortierite prismatic parallel to 
b or a * 
•Mon. U. S. G. S. xxix, p. 28, 1898. 
