KEWEENAW POINT AND ST. CROIX ROCKS COMPARED. 239 
sandstones and the bedded melaphyrs and amygdaloids upon which they lie 
being so plainly indisputable, the latest advocate of the old idea of the con- 
temporaneousness of these sandstones with the copper-bearing or Kewee- 
naw rocks has been driven to question the correctness of the identification 
of the bedded diabases and amygdaloids of the Saint Croix Valley with 
those of Keweenaw Point.’ It is therefore proper that I should insist 
here that this identification is also indisputable; and that it is so because of 
the absolute identity in nature and structure of the rocks of the two regions, 
and because the Keweenaw belts have been followed continuously from the 
eastern end of Keweenaw Point to the Saint Croix River. 
In support of the first of these assertions, I have to advance the follow- 
ing facts. The predominant fine-grained basic rocks of the two regions are 
so completely the same in mineral composition, even to the alteration-pro- 
ducts, that thin sections of rocks from the two districts placed side by side 
are not distinguishable from one another. The only approach to an excep- 
tion to this statement is the somewhat greater prominence of prehnite as an 
alteration-product on Keweenaw Point than on the Saint Croix” The 
rocks of the two regions present precisely the same amygdaloidal, pseud- 
amyegdaloidal, and compact phases. The amygdules are made of the same 
minerals in both, associated in the same ways. Native copper occurs in 
the Saint Croix Valley in the same manner, and with the same associates 
as on Keweenaw Point. Here and there an exposure may represent a dike 
so far as can be perceived, but almost everywhere the Saint Croix Valley 
rocks present precisely the same bedded structure as seen in those of Ke- 
weenaw Point. This is displayed, not only in the common step-like con- 
tours of the exposures, but the individual beds may be readily separated 
from one another, each bed often showing sharply marked its upper 
1‘“ Notes on the Iron and Copper Districts of Lake Superior,” by M. E. Wadsworth. Bulletin of 
the Museum of Comparative Zoology at Harvard College. Whole series, Vol VII. Geological series, 
Vol. I, No. 1, p. 107. 
2Compare R. Pumpelly, Geology of Wisconsin, Vol. III, p. 36. ‘‘While the absolute identity of 
the diabases and melaphyr and of their varieties and amygdaloids, and of the interbedded porphyry 
conglomerates of the Wisconsin area with those of Keweenaw Point is evident, I am struck by the com- 
parative scarcity in the former, of one of the most important forms of alteration that abounds in Michi- 
gan; I have found in the four collections but one instance of change of feldspar to prehnite.” With 
regard to this it should be said that the specimens from Wisconsin examined by Pumpelly included 
very few from the uppermost belts of the Lower Division, which carry prehnite much more commonly. 
