76 '/7/e ^l htericaii (ieolo(ji.sf. August, 1895.. 
3. The Kcwt-eiiawan consists essentially of eruptive mate- 
rials, but toward the top has interstratified fragmental beds, 
such as conglomerates, sandstones and shales, the aggregate 
thickness being from 40,000 to 45,000 feet, three-fourths of 
which is igneous material. The lowest member consists of a 
great thickness of diabase and allied igneous rocks. The 
conglomerates consist almost wholly of porphyry pebbles, 
which cannot be certainly referred to their native sources. 
One conglomerate is 1,200 feet thick. The trap beds are tilted 
sometimes to angles of 45 degrees and greater, the immediately 
overlying sandstones having a conformable dip. After the 
Keweenawan there was an interval of erosion. The whole 
period of the Keweenawan was accompanied by subsidence of 
the region of lake Superior. This is proved by the enormous 
thickness of the deposits. The subsidence was also accom- 
panied by rtexure and faulting. 
4. After the Keweenawan was elevated and broken there 
supervened a period of erosion, during which the region was 
a land area. This was the time of the Taconic or Lower 
Cambrian, the sediments of which are found on the Atlantic 
seaboard. Then the sea returned slowly over the region, de- 
positing a non-conformable sandstone u] > n the upturned beds 
of the Keweenawan. 
5. This non-conformable sandstone is the Potsdam, and 
its distinctness from the Keweenawan is not a matter of 
doubtfid opinion. Its accumulation was also accompanied by 
continued subsidence. 
• From Nos. 1 and 2 of these general conclusions the writer 
has no reason to express essential dissent. He wishes here 
only to call attention to the discriminating foresight of Prof. 
Chamberlin in not accepting the expanded Huronian of the 
Canadian geologists, though it was advanced by Irving and 
Brook's in the same report. The strata which Prof. Chamber- 
lin accepts as typically "Huronian" are those of the Penokee 
range, w^hich, except the cherty limestone, are by Van Hise- 
classed as "Upper Huronian" in the correlation paper ArcJiean 
(i)i(l AIfio»ki(()i, and which at a later date Irving considered, 
rightly, the equivalent of the Animikie and of the original 
Huronian. The principal correction in the Huronian of Prof. 
( haniberlin, which later study has shown to be necessaiy, is- 
