Gabbroid Rocks of Minnesota. — WinchelL 205 
The ratio of Fe to Mg is thetrefore very nearly 5:4, and 
the mineral is accordingly an "hyalosiderite" remarkably rich 
in iron. Qualitative tests show such slight traces of TiO.^ that 
this result may be considered sufihciently exact since the pro- 
portion of ilmenite attacked must have been extremely small. 
Magnetite varies much in abundance; it is usually titanifer- 
ous as shown by the leucoxene found by Bayley, and the titan- 
ite, well crystallized, and highly birefringent found by the 
writer. Octahedral parting occurs uncommonly. Decompo- 
sition similar to that observed in the plagioclasyte occurs as 
well as the alteration to titanite. The first change is to an 
amorphous dull black substance, not at all metallic in lustre; 
this is probably amorphous magnetite; to this succeeds, in the 
Birch lake diabase, well crystallized pyrite with very character- 
istic brass-yellow reflection. Mesogenesis of biotite between 
the magnetite and feldspar is quite common. 
Magnetite is occasionally automorphic, but it is oftener 
of very irregular shape; it is often enclosed by the pyroxene 
and feldspar, and nearly as often occurs in irregular masses 
filling the interstices. A peculiar pegmatitic intergrowth of 
magnetite and augite occurs in the olivine gabbro from Birch 
lake. (See Plate VII, Fig. 2.) 
An opaque black mineral which seems to give the blue- 
black metallic lustre of magnetite occurs in the olivine diabase 
from Pigeon point as a minute inclusion of irregular outline 
in clinochlore in which it causes a very marked pleochroic 
halo, which is peculiar in showing a lighter circle inside the 
darker one." (See Plate XVII, Fig. 5.) The refringence and, 
apparently, the birefringence are increased by the halo. 
Apatite is the only other original mineral reported by Bay- 
ley; it occurs in the usual colorless acicular crystals with hexa- 
gonal cross sections. It is strictly uniaxial and negative with 
parallel extinction and negative elongation. The birefringence 
is very weak (.004). Very minute dust-like inclusions, prob- 
ably magnetite, are often abundant. These are sometimes 
arranged along curving planes. Liquid inclusions are un- 
common; they sometimes contain one or more particles of 
magnetite(?). Apatite often causes marked halos in brown 
and green hornblende and in biotite. 
Allanite occurs in minute crystals, often rounded, or even 
