THE GEOLOGICAL SECTION OF MICHIGAN 687 
glomerate under the general head ‘‘Trap rocks” and understood that 
“strictly in their chronological order” they came after the ‘“‘meta- 
morphic” slates and quartz rock. The Lake Shore traps were 
thought of as intrusive dikes, so that he could group the Greater and 
Outer Conglomerate together under the head “conglomerate” and 
the rest of the Keweenawan as “‘mixed conglomerate and sand rock.” 
Foster and Whitney followed his divisions in their mapping, including 
them as the lowest divisions of the Silurian (which, as in the early 
editions of Dana, was understood to go down to the Azoic and include 
the Primordial) and as intimately associated with the Lake Superior 
sandstone, so much so as not to need any separate formation name. 
It has already been mentioned that the latter has been referred to 
various ages from Triassic back. Logan called them the ‘Upper 
copper-bearing”’ rocks; Brooks and Pumpelly the Latin equivalent 
‘“‘Cupriferous,” and considered the formation conformable to the 
Huronian, but covered unconformably by the Lake Superior sand- 
stone, and likely to be more allied to the former than the latter. 
Irving introduced the term Keweenaw(an), and in Monograph V, 
p. 24, recognized an unconformity (disconformity) of the Keweenawan 
and Huronian, as well as an unconformity above, especially with the 
Mississippi Valley Cambrian sandstones. Neither of the unconform- 
ities do we doubt. But they appear to be with basal Keweenawan 
beds disturbed by coeval volcanic activity and faulting, and seem to 
us quite comparable with inter-Keweenawan phenomena, while the 
Lake Superior sandstone and the Upper Keweenawan appear to us 
closely associated not merely lithologically but stratigraphically.? 
We prefer, therefore, for the present, to place the Keweenawan in 
the Cambrian to express that fact. If we could but be sure that there 
was indeed a universal cycle of sedimentation and rhythm of geologi- 
cal activity, by which, all the world over, crustal rearrangements and 
volcanic activity took place simultaneously, while at other times 
atmospheric conditions universally favored the deposit of limestones 
or black shales, there would then be more of a temptation to compare 
our column with the type column in Great Britain and see in the 
Longmyndian red sandstone the correlates to the Ke- 
and volcanics weenawan. 
eee oe oe oo 
t See Annual Report, Board of Geological Survey of Michigan, 1904, p. 143. 
