186 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
Range rocks thus given shows that in this range we have the source of all 
the different kinds of porphyry pebbles and porphyry detritus which char- 
acterize the conglomerates and sandstones of the series, viz: felsite without 
visible quartz or feldspar (Mount Houghton belt), augite-syenite (Mount 
Bohemia), quartzless porphyry (old Suffolk location), and true quartziferous 
porphyry with large porphyritic quartzes and orthoclases (Torch Lake Rail- 
road). The points named are of course not the only ones in which several 
kinds of porphyry respectively occur, but they serve to show that in these 
lower belts we are to find the source of all the porphyry detritus—a fact 
which has never been recognized before. It is shown elsewhere that these 
original porphyries characterize the formation throughout its entire extent 
around Lake Superior. Moreover, the variation of the conglomerates as to 
predominating pebbles, when followed along the strike, is to be connected with 
the varying characters of the original porphyries when followed in the same 
direction. 
The lowest horizon reached east of Kagle River, or indeed anywhere 
on Keweenaw Point, would appear to be in the neighborhood of Gratiot 
Lake. The highest horizon east of Eagle River is reached in the outer 
conglomerate to the east of Copper Harbor. Between these two limits the 
eastern part of Keweenaw Point appears to be made up as indicated in the 
following summary : 
UPPER DIVISION OE THE KEWEENAW SERIES. 
Thickness in feet. 
The Outer Conglomerate; porphyry-conglomerate and sandstone, about...... 1, 000 
LOWER DIVISION OF THE KEWEENAW SERIES. 
“The Lake Shore Trap; very plainly bedded fine-grained diabases, strongly 
marked amygdaloids, and one or more thin porphyry-conglomerates; about 1,500 
The Great Conglomerate; porphyry-conglomerate and sandstone............- 2, 200 
Marvines Group “ce”; plainly bedded and separable fine-grained diabases, with 
strongly marked amygdaloids, predominatingly calcitic; and some 850 to 
900 feet, in all, of interstratified sandstones..:...-...-.....----.-.-.--- 1, 417 
Marvine’s Group “b,” or the Ashbed Group; made up mostly of thin, fine-grained : 
diabases, which vary a good deal in appearance, but are generally provided 
with distinct amygdaloids; including some beds of the peculiar type known 
as ashbed-diabase; also several scoriaceous amygdaloids, being intermin- 
gled sandstone and amygdaloid; also one thin sandstone seam......-.-... 618 
Marvine’s Group “a”; made up of relatively heavy beds without strongly devel- 
oped amygdaloids; including one thin seam of sandstone........---.-..- 925 
