ROCKS OF MICHIPICOTEN ISLAND. 343 
south of the harbor on the south side of the island, I find to be a highly 
siliceous felsite, closely resembling and plainly belonging with the red rock 
of which Mount Houghton, on Keweenaw Point, is formed, which makes up 
much of the central mass of the Porcupine Mountains, and which forms so 
many of the red cliffs of the Minnesota coast of Lake Superior. The resem- 
blance is both macroscopic and microscopic; while the peculiar “festooned 
and wrinkled” markings, “‘composed of very thin close-fitting lamin, with 
a ligneous aspect,” noticed by Logan, are precisely what I have repeatedly 
described in the foregoing pages as characterizing similar rocks in so many 
places in the western half of the Lake Superior Basin. These markings are 
due doubtless to a viscous flow, and are much the same as are found to 
characterize the modern rhyolites. The high stratigraphical position of this 
felsite, which is described by Logan as plainly dipping with the rest of the 
Michipicoten series, is particularly worthy of notice, since it places these 
acid rocks at a higher horizon than elsewhere in the Lake Superior Basin. 
Of the peculiar resinous-looking rocks, which, under the name of 
pitchstone and pitchstone-porphyry, Logan describes as showing all along 
the south shore of the island, I find several specimens in Macfarlane’s col- 
lection. One of these specimens,’ labeled by him ‘compact melaphyr,” 
presents a nearly aphanitic, dark-gray rock, with a conchoidal fracture, and 
without porphyritic ingredients. It bears a strong resemblance to the ash- 
bed traps of Keweenaw Point. In the thin section this resemblance is borne 
out completely, the rock proving to consist of predominant tabular oligo- 
clases, with augite in the characteristic irregular grains whose contours are 
not determined by the feldspars. Magnetite and some non-polarizing ma- 
terial, which is taken to represent residuary magma, are also present. The 
rock is thus, according to the Rosenbusch nomenclature, a diabase-porphy- 
rite. 
Another specimen,’ also called melaphyr by Macfarlane, is aphanitic, 
of a dark chocolate-brown color, has a conchoidal fracture, and shows no 
- porphyritic ingredients. The thin section of this rock proves it also to be 
a diabase-porphyrite, the ingredients being the same as in the last, the only 
perceptible differences being that in this rock the tabular feldspars are two 
1Macfarlane’s No. 2. 2? Macfarlane’s No. 5. 
