ROCKS OF MICHIPICOTEN ISLAND. 345 
tion the brownish-stained matrix is seen to be composed in a considerable 
measure of unindividualized material (large areas remaining dark between 
the crossed nicols) exceedingly minute tabular plagioclases, occasional minute 
brightly polarizing augite points, and magnetite particles in the groundmass. 
The still lower strata which form the bulk of the thickness present in 
the island are not so well represented in the collection. One specimen’ from 
the copper mines on the north side of the island, called by Macfarlane mela- 
phyr, is fine-grained, plainly crystalline, and has a rough fracture, and a dark 
purplish-gray color, mottled with still darker shades. Its aspect is identical 
with that of many of the finer-grained, luster-mottled, olivinitic rocks to which 
Pumpelly has given the name of melaphyr. This resemblance is fully borne 
out by the appearance of the thin section, in which the characteristic rela- 
tively large augites include numbers of tabular plagioclases, while the abun- 
dant olivines, wholly altered to green and brown substances, are crowded 
with the magnetite into the interspaces of the augites. A number of large 
porphyritic plagioclases occur in the section, an unusual thing for this class 
of rocks elsewhere in the Keweenaw Series. 
The copper-bearing “vein” at this place is evidently merely one of the 
usual altered cupriferous amygdaloids, and is, according to Macfarlane, 
almost identical with the cupriferous amygdaloid of the Pewabic and Quincy 
mines, Portage Lake. It has been traced for a considerable distance. 
Overlying this cupriferous bed is a rock which is represented in the 
Macfarlane collection. This rock? is completely aphanitic, of a dark-gray 
color and highly conchoidal fracture, and shows as porphyritic ingredients 
only rare black crystals of augite. The thin section proves it to be a typical 
diabase-porphyrite, with a predominating isotropic, pinkish-tinted, and cloudy 
base, in which may be recognized, with a high power, excessively minute 
tabular plagioclases, larger but still minute magnetite particles, and rare 
and very minute brightly polarizing particles, probably belonging to augite. 
The porphyritic augites are largely replaced by a greenish alteration- 
product. Aphanitic black rocks of similar appearance to that last described 
form, according to Macfarlane, a large proportion of the lower half of the 
Michipicoten section. Rocks of this character show all along the west shore 
of the island to its western point. 
1 Macfarlane’s No. 1. 3Macfarlane’s No. 9. 
