174 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
mixed brown and green portions, both hard, the brown aboundiug in amygdules, from 
one-third inch in diameter down, chiefly of prehnite; often of prehnite as an outer 
member, and a central filling of quartz in some, in others calcite. The green contains 
fewer apparent amygdules. Thin sections of the brown part show the sharp outlines 
of comparatively large porphyritic feldspar crystals, and of countless long slender 
feldspar microlites separated by an opaque brown substance. ‘These feldspar forms 
are now occupied by brilliantly polarizing aggregates of prebnite. Splinters of this 
brown matrix fuse in the flame of an alcohol lamp. Some of the feldspar forms con- 
tain a large amount of soft, light-green, seemingly amorphous mineral, which is, prob- 
ably, pseudomorphous a/ter prehnite; the rest of the pseudomorph in these cases seems 
to be quartz. The amygdules have very sharply defined contours, and form brilliantly 
polarizing aggregates of prehnite. Quartz occurs in seams which cut through the preh- 
nite of the matrix, and of the amygdules. An examination of thin sections of the green 
parts shows that they are derived from the brown. They consist still to a great extent 
of prehnite, and many pseudomorphs of this after the feldspar are visible; but it 
is everywhere more or less changed to the light-green, soft substance (of which some 
was seen in the brown variety), and considerable areas of the field are wholly changed 
to this substance, which is thoroughly cut up by curving cracks of irregular shape and 
size, which are evidently due to contraction, and are now filled with quartz. But 
little of the brown staining seen in the brown variety is present here; the iron oxide 
causing it has, perhaps, gone towards forming the green earth-like alteration-product 
of the prehnite. Splinters of this variety show under the loupe by transmitted 
light, nearly opaque, light-green portions, separated by transparent white." 
Further down in this division of this series the beds are not well ex- 
posed, but finer-grained, darker, and more evenly colored beds seem to 
prevail. Near the base of the group the beds are heavy and more like those 
near the summit. The bottom bed (90) is a hard, fine-grained, bluish- 
black, typically luster-mottled melaphyr. 
Marvine’s next subdivision (beds 91 to 108, inclusive), in descending 
order, which we may appropriately call the Greenstone Group, since its great 
basal bed forms the well-known Greenstone Ridge, he regards as a series 
of diorites, but Pumpelly has shown that all are pyroxenic. These beds 
form a massive group, sharply contrasted with all the rocks above them, 
and with those immediately below. The thickness is 1,200 feet, in heavy 
beds from 20 to 150 feet or more in thickness. There are no intercalated 
amygdaloidal bands. Excepting a very massive layer at the base and a 
thinner one upon the top, all the interior two-thirds of the group have a 
moderately coarse-grained texture, especially so as compared with the 
adjoining rocks. 
1 Proceedings American Academy Science, Vol. XIII, p. 280. 
