198 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
as if rounded or eaten into by the matrix. Cores of brightly polarizing 
augite remain in many of these crystals, and where the black material is 
very thin it transmits a red light. The crystals are thus seen to be augite, 
with a ferritic alteration. The silica content is 65.8 per cent. Evidently we 
have, then, in the Stannard’s Rock reef, one of the felsite belts of the Ke- 
weenaw Series which have furnished so many of the interbedded conglom- 
erates with pebbles, the sections of many of these pebbles appearing pre- 
cisely like that of the Stannard’s reef rock. The shortness of the reef coin- 
cides perfectly with what is known of these felsite belts elsewhere. The 
belts themselves are often relatively short, and, moreover, tend to stand up 
in just such summits, some parts of the belt resisting erosion more than 
the rest. Mount Houghton is just such a summit, and it would not be a 
bold speculation to consider Stannard’s Rock and Mount Houghton as parts 
of the same belt, or rather as belonging near the same general horizon. 
However that may be, Stannard’s Rock certainly marks for us the course 
of the Keweenaw Point Range. At the end of the point the beds are trend- 
ing directly towards Stannard’s Rock, whose own curving trend coincides 
perfectly, as subsequently shown, with the general structure that I have 
worked out for the whole Keweenaw basin. 
Section Il.—THE REGION BETWEEN PORTAGE LAKE AND THE ONTON.- 
AGON RIVER. 
In the 40 miles that intervene between Portage Lake and the Onton- 
agon River the exposures are not abundant until the latter stream is nearly 
reached. Evidently enough, however, the conditions observed at the for- 
mer place continue throughout most, if not all, of the distance. There is 
the same southeastern lowland, with its horizontal sandstone; the same single 
Trap Range, with abrupt southern and gradual northern slopes, composed 
of the same strata, dipping at the same high angle towards Lake Superior, 
and having about the same thickness; and the same northwestern lowland 
bordering the lake and occupied by the sandstone of the Upper Division of 
the series. Near the Ontonagon River the main mass of sandstone, black 
