'Jlie Keiree/Ktiroji. — Winchell. 77 
the separation of a part of the iron ores from the Penokee 
series and the relegation of them to the greenstone and per- 
haps to other horizons of the Archean. This would also ne- 
cessitate the recognition of a non-conformity at the base of 
his Penokee series, by which that series would be likely to 
occur transversely and non-conformably on all the parts of 
the Archean, including many of the so-called diorytes, which 
are really phases of the greenstones. There is still much 
doubt as to the true place of the basal limestone of Chamber- 
lin's Penokee series, since Van Hise has latterly included it in 
the Lower Huronian, thus placing it below the great non-con- 
formity instead of above it where it had uniformly been 
classed before.* 
We desire, however, to call attention to Nos. 3, 4 and 5 of 
these conclusions, and to examine the evidence on which they 
are based, as thc}^ are the same as expressed by Messrs. Wal- 
cott and Van Hise in the two "Correlation papers" to which 
reference has been made. 
No. 3, of Chamberlin's summary, defines the Keweenawan 
and gives its essential structural relations. As to the nature 
of the Keweenawan, later examinations have rendered neces- 
sary a slight reconS" action of this description. Instead of 
having an enormous diabase mass at its base, or a gabbro mass, 
the base consists of fragmental materials. Prof. Irving refers . 
to this fact in his later work on the Keweenawan, f saying that 
layers of conglomerate and sandstone extend nearly to the 
base, the conglomerates being generally of acid eruptive rock,, 
such as felsytes, quartz-porphyries and augite syenites. He 
refers also specially to the light-colored and marly sandstones 
of the region of Black and Nipigon bays, on the north shore, 
which lie non-conformable upon the Animikie slates and have 
a thickness of about 1,300 feet according to Robert liell. 
These he considers as in the base of the Keweenawan. If he 
is correct in this there is abundant reason to put the Kewee- 
nawan age in the midst of a great sandstone epoch. But we 
shall find evidence to believe these Black Bay and Nipigon 
*See: Penokee Iron-bearing series of Michigan and Wisconsin. 
Monograph xix, U. S. Gcol. Sur., p. 473, 1892. 
fThe copper-bearing rocks of lake Superior. Mon. v. U. S. Geol. 
Survey, pp. 152, 155, 1.5G, 188.3. 
