1 66 TJie American Geologist. September, i90o 
base by assuming the ophitic structure. Magnetite, olivine, 
augite, and labradorite are always present as primary con- 
stituents, and apatite can usually be found also, ordinarily in 
extremely small amount. The apatite usually crystallized very 
early, but its relation to the magnetite has never been ob- 
served ; it is often enclosed in good crystal form within the 
augite, less commonly it is surrounded by labradorite, or some 
one of the other minerals. Since the augite alters sometimes 
to biotite, apatite is often found enclosed in the latter mineral. 
The magnetite usually shows no good crystal outlines, but in 
the few cases where they occur the iron oxide is clearly older 
than the other minerals. The augite in particular has been 
observed molding itself sharply around clear cut contours of 
magnetite. (See plate XVII, figure i.) The iron oxide is 
oftenest in irregular grains and masses, frequently between 
plagioclase anhedra, but also occurs included within them. It 
is practically never absent, but is seldom abundant. There is 
very little doubt that the fine acicular inclusions in the feld- 
spar, as well as dust-like particles which are usually most 
abundant in the plagioclase, but can frequently be found also 
throughout the rock, are all to be referred to magnetite, usual- 
ly titaniferous. Olivine is very rarely absent, and may be quite 
abundant. Its period of crystallization was probably ended at 
or before the time when the pyroxene began to solidify, and it 
served as a sort of support upon or against which the augite 
crystallized. Thus the olivine is often entirely surrounded by 
a rim of pyroxene which may finally grow out in some direc- 
tion and form a distinct crystal (not a mere rim) though it is 
usually oriented like that around the olivine. That these rims 
are not "reaction rims" is well shown by their development, 
thus, away from the olivine, as pointed out by Bayley*. Oc- 
casionally the olivine is apparently later than the augite. (See 
Plate XVII., figure i.) 
The pyroxene is in general automorphic toward the feld- 
spar in which it is frequently enclosed. This rule is subject, 
however, to distinct exceptions, where the augite occurs in 
irregular patches or triangular areas between the labradorite 
grains and is clearly younger than the latter. Such exceptions 
*W. S. Bayley: The basic massive rocks of the Lake Superior 
Region: Jour. Geol. 1893, I, p. 706. 
