Gabbroid Rocks of Minnesota. — Wi?ichelL 171 
in acicular form. It occurs in pegmatitic intergrowth with 
the pyroxene. (See plate VIII., figure 2.) By reflected Hght 
the octahedral parting can often be discerned. The mineral is 
strongly magnetic, but usually has no polarity. Its color is 
iron-black, with bright blue-black metallic lustre. 
Though the mineral undoubtedly often contains titanium, 
it has never been seen to alter to leucoxene. 
In direct contrast with the rarity of crystal form in magne- 
tite is its frequency in apatite. Indeed it is rare to find apatite 
except in well formed, sharply limited crystals. These are 
hexagonal prisms, usually rather short, and terminated some- 
times by the basal plane, and sometimes by pyramidal faces. 
Twinning has never been seen, and cleavage is rare and in- 
distinct; it occurs, however, parallel to the base, and still more 
imperfectly in the large crystals, parallel to one or more of the 
prism faces. 
Microchemical tests have shown the presence of phosphoric 
acid and calcium. 
Apatite occurs generally in small crystals sparsely distrib- 
uted; many sections contain none at all; it is so rare as to be 
practically without any effect upon the composition of the 
rock as a whole. It may be enclosed by any of the other min- 
erals, though it most commonly occurs in the labradorite and 
augite. 
Apatite sometimes occurs causing no halo in the surround- 
ing mineral, whether that be labradorite, augite, brown or 
green hornblende, or biotite. In such cases the refringence of 
the apatite is always higher than ?ig of the biotite. In other 
cases apatite always causes a halo when surrounded by biotite 
or hornblende. Finally it sometimes happens that in a single 
thin section apatite causes a halo in some biotite crystals and 
does not in others. A thin section of the Birch lake gabbro 
shows this peculiar condition. When the apatite causes no 
halo, apatite ?ig > wg and wm biotite. 
In another case a transverse section with good outline 
causes a marked greenish brown halo in brown biotite (see 
plate XVII, figure 3). In this case: 
Apatite «g > ftp biotite, difference considerable. 
Apatite «g <«m biotite, though nearly equal. 
The refringence («g and um) of the biotite sometimes re- 
