BASIC ORIGINAL ROCKS. 37 
It will be seen at once that all of the kinds named are very closely 
related. Except one extreme phase—the anorthite-rock—they are all pla- 
gioclase-augite rocks (the diallage being but a phase of ordinary augite) 
and all carry magnetic or titanic iron, while olivine is a common ingre- 
dient. The differences consist only in variations in coarseness of grain, 
relative amounts of the several ingredients, presence or absence of olivine, 
presence or absence of unresolvable base, presence or absence of ortho- 
clase, variations produced by metasomatic changes, and variations in texture 
from granular to vesicular (amygdaloids). All of the kinds, though dis- 
tinct enough in the field, are in fact, lithologically considered, but phases of 
one kind of rock, for which usage has not established amy common name. 
The term “ basalt,” restricted by Rosenbusch to the younger equivalents of 
the pre-Tertiary olivine-diabase and melaphyr, has been much used with a 
more extended signification, and might, perhaps, be not improperly used as 
a general term for all plagioclase-augite rocks, young and old. 
BASIC ORIGINAL ROCKS OF THE KEWEENAW SERIES. 
I. COARSE-GRAINED. 
1. Gabbro and diabase; olivine-gabbro and olivine-diabase; all free from ortho- 
clase. 
2. Orthoclase-bearing gabbro. 
3. Hornblende-gabbro. 
4, Anorthite-rock. 
II. FINE-GRAINED. 
5. Diabase of the ‘“‘ordinary type.” 
6. Olivinitic fine-grained diabase and melaphyr. 
7. “ Ashbed”-diabase and diabase-porphyrite. 
8. Amygdaloids (vesicular diabase and melaphyr). 
COARSE-GRAINED BASIC ROCKS. 
Orthoclase-free diabase and gabbro, olivine-diabase and olivine-gabbro.—The 
rocks included here have a prevailing very dark-gray, often black, shade. 
More rarely they are light-gray, when the plagioclastic ingredient becomes 
greatly predominant, as is apt to be the case in the coarsest kinds. Not 
unfrequently a brownish film (ferrous oxide) over the shining black augite 
produces a resinous hue, and a somewhat similar effect is occasionally 
produced by a relatively large proportion of olivine. The texture is a 
