246 R. D. Irving—Diwisibility of the Archean. 
formation. On this view, the structure of the Marquette 
region becomes plain. The slate series above the greenish 
schists, in the main composed of relatively little altered rocks, 
was originally built up upon a basement composed of granite, 
gneiss, and these greenish schists themselves, and subsequently 
was pushed into trough-like forms by lateral pressure. Denu- 
dation then brought matters to their present condition. 
To support this view we have many of the same arguments 
as advanced in the case of the Penokee region. Discordance 
various other points in the Lake Superior Archzan Region. 
_ There is, indeed, one way in which we can avoid the conclu- 
sion which, as it seems to me, has thus been forced upon us 
viz: that the Archean of this region is divisible into at least 
two wholly separate members. This is by throwing the newer 
of the two divisions into the Cambrian and calling only the. 
lower one of the two Archwan.* By so doing, however, we 
* This is essentially vans N. H. Winchell would do in referring all of the 
western Huronian rocks t aconic of Emmons (Amer. Naturalist, October, 
1884, pp. 984-1000). With the Taconic system, and, indeed, wi 
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“Taconic question” I have little acquaintance, but I cannot feel any great confi- 
i e parallelization of formations so distant from one another as the 
r nd the “Taconic,” on exclusively lithological evidence. 
However this may be, there is, in Professor Winchell’s general arrangement of 
the crystalline rocks of the northwest, much with which I can cordially agree. 
