240 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
vesicular and lower compact portions. Moreover, where the dip is high and 
the exposures are large, as on the Snake and Kettle rivers of Minnesota, 
there is to be seen a continuous series of beds, in all many hundred feet 
thick and in every respect similar to the alternations which obtain on 
Keweenaw Point. The same interstratified porphyry-conglomerates and 
sandstones are met with in both regions, and in both regions carry at 
times native copper. Interbedded original felsitic porphyries also occur in 
both regions. 
In support of the second assertion, as to the actual continuity of the 
Keweenaw Point and Saint Croix rocks, I have to say, in the first place, that 
the evidence of this continuity is precisely the same for the distance between 
the Montreal and the Saint Croix, as for that between the Montreal and 
Keweenaw Point, or even the distance between the eastern part of Kewee- 
naw Point and its western portion at Portage Lake; that the continuity has 
never been disputed for the two latter distances; and that it should there- 
fore be accepted at once for the first-named distance. The evidence for all 
the distance between Keweenaw Point and the Saint Croix is just as strong 
as that ever appealed to to prove the continuity of geological formations 
anywhere, save in those very rare and exceptional regions where the rocks 
are completely bare. This evidence consists in the frequent recurrence, at 
short intervals, of the same kinds of rocks, with the same structure and 
stratigraphiecal arrangement; and such evidence is forthcoming in the present 
case. From Keweenaw Point to the Saint Croix, the formation has been 
traced mile by mile with a constant recurrence of precisely the same bedded 
basic rocks, with the same amygdaloidal and compact portions to the beds, of 
the same associated felsitic porphyries, of the same interstratified porphyry- 
conglomerates, and of the same native copper in veins, altered amygdaloids 
and conglomerates. The same division of the series into a Lower or prevail- 
ingly eruptive member, and an Upper or detrital member, is also everywhere 
present. From Keweenaw Point to the region of Long Lake, some even 
of the subordinate members are recognizable as continuous. For the par- 
ticulars of this evidence, I refer to the detailed descriptions of Foster and 
Whitney’s report, of Vol. III of the Geology of Wisconsin, and of the 
present work; to the United States Land Office township plats; and to the 
