A RdfioiKil Vieir (if the Keu'eeiifi ii-dii. — W'incJiell. 153 
erosion interval between the Keweenawan sandstones and the 
horizontal sandstones — such an interval as would allow t-he 
lapse of the time involved in the Taeonie, or Lower Cambrian, 
while the region of lake Superior was dry land. During such 
an interval the upper Keweenawan sandstones would have 
been entirely swept away, or so hardened that they would 
show marked litholog-ical contrasts with the ''horizontal sand- 
stones." 
What has now been said relating to the Keweenawan is 
mainly of the nature of destructive and controversial criti- 
cism of the A'iews which are set forth in Bulletins 81 and 86, 
of the United States Geological Survey, touching the nature 
and structural relations of those rocks. It sometimes becomes 
necessary to enter upon wholesale re-examination of the evi- 
dences on which hypotheses are founded. It is proposed now 
to construct the Keweenawan in the light of some newly dis- 
covered facts, and to indicate its stratigraphic place. 
The erirpflve racks- irhlch have been iiirlvded In Jfir/n'i/tni, 
Wisconsin and Miniiesofa iit the Keireeiiairtm coii.sisf of two 
widely different series, of widely separated (kjcs. This state- 
ment is based on facts observed in Minnesota, mostly not yet 
published in detail, but it is believed to be equally applicable 
in Wisconsin and Michigan. The brief letter of Dr. U. S. 
Grant published* in June, 1S94, expresses the key to this sep- 
aration, viz., the Animikie was upturned by eruptive disturb- 
ance, and changed to quartz-porphyries and probabl}^ augite- 
syenites prior to the Keweenawan. Along with this disturb- 
ance there issvied from greater depths in the earth great 
quantities of gabbro and allied eruptives. This may be fully 
understood by consulting the iiiiijortant work of Dr. W. S. 
Bayley on Pigeon point, f without further specification. The 
eruptive red rocks which Bayley has described can be traced 
westwardly to Brule lake, and still further west and to Du- 
luth. They are not of Keweenawan age, thougli so mapped 
and described by Irving. They iire everywhere associated 
with modified conditions of the Animikie and with coMrsely 
crystalline basic rock. Indeed, the great gabbro, or anorthosyte, 
^American Geolocjist, vol. xiii, p, 4.37, 1894. 
tThe eruptive and sedimentary rocks of Pifreon point, Minn., and 
their contact phenomena. Bulletin 109, U. S. (Jeol. Surv., 18!K5. 
