18 The American Geologist. July, i895 
eel Huronian ideas of the Canadian survey, and attempted a 
delineation of tlie stratigraphic succession of the rocks of 
that system as exemplified in the Marquette district, making 
nineteen stratigraphic parts, to which he subsequently added 
one more, an eruptive granite supposed to be younger than 
all the others. He says nothing of the duplicate nature of 
the copper-bearing series, brought out b}^ Logan in 1863, but 
only describes Logan's "upper volcanic group." He institu- 
ted, however, an important distinction in this series, as it had 
before been described, viz., he separated the traps and their 
tilted sandstones from the horizontal sandstones which he 
called Silurian. He does not seem, however, to be very posi- 
tive about the validity of this distinction, since at his typi- 
cal locality for examining the relations of these rocks* he 
calls attention to sandstones '•'■apparently Silurian," which had 
a dip amounting to 45° toward the south. He says: 
So far as my obsprvalion has extended, this rule is general: that is, 
no Lake Superior sandstone whicli is unmistakably Lower Silurian has 
ever been found in any position other than liorizontal: and no rock 
which was unmistakably of the Copper series has been seen which was 
not considerably lilted. The fact tiiat certain sandstones belonging to 
the Copper series ;irc \-ery similar, if not lilholoiiically identical with 
some of the Lower Silurian sandstones, has helped to complicate the_ 
question. 
Subsequentl_y he united with Hunt in the name Keweenawan 
for the traps and their associated amygdaloids and inter- 
stratified sandstones. f It will be noted that the use of the 
qualifying word "unmistakably," byALij. Brooks, on each side 
of tills dividing datum, in the classification which he adopts, 
leaves a wide margin of uncertainty. It is within this mar- 
gin that the crucial facts on which such a distinction should 
rest, ought to be sought for ; for on close inspection it might be 
found that, on either side, those strata whicli are assumed un- 
mistakably to be one or the other, show characters of dip or other 
structural relations which would make them doubtful. Others 
might be doubtful on account of lithology. Thus the datum 
of distinction might disappear in thoroughly exploring tiie 
unknown interval of uncertaint}' existing between the unmis- 
♦Geological Survey of Michigan, vol. i, p. 185, 1873. 
fCompare U. S. Grant: The name of the copper-bearing rocks of 
lake Superior, American Geoi.ogist, vol. xiv, pp. ] 92-194, March, 1895. 
