RELATIONS OF THE KEWEENAWAN AND HURONIAN ROUKS. 403 
differences. These rocks occur in both groups in great interbedded flows, 
but the amygdaloidal or vesicular upper portions are wanting, or nearly so, 
in the Huronian, while the occurrences as dikes or cutting masses is far 
more common in the Huronian than in the Keweenawan. This is a fact 
evidently indicating, as shown before—since the dike rocks are often iden- 
tical with and commonly very closely related to those that occur as flows 
in the Keweenawan—-that in the fissures now filled by the Huronian dikes 
we find the vents through which came both Huronian and Keweenawan 
flows. 
Passing from the basic eruptive rocks of the two groups to the acid 
kinds, the similarity does not hold. We look in vain in the Huronian 
throughoutthe entire Lake Superior region for the great flows of red felsite and 
quartziferous porphyry which constitute so marked a feature of the copper 
series. Nearly the same may be said for the red augite-syenite and granitic 
porphyry which form so great masses in the Lower Keweenawan, although 
it appears probable from Norwood’s statements’ that similar rocks on a 
smaller scale occur in the Huronian (Animikie) of the Pigeon River region 
of the North Shore. Logan’s descriptions of the west shore of Lake Huron 
suggest also the possibility of the occurrence of such rocks there. 
Between the distinctly bedded or sedimentary rocks of the two groups 
the contrast is strong. The Keweenawan red sandstones and shales have 
nothing in common with the quartz-slates and quartz-schists of the 
Huronian, even when the latter are hardly more than sandstone and clay- 
shale. The two groups are thus linked by the basic rocks of each, the 
basic eruptions having apparently begun in the later Huronian and con- 
tinued uninterruptedly into the Keweenawan. They are separated by the 
absence or relative rarity of the acid eruptives in the Huronian, and by 
the strong contrast between the sediments of the two groups. That the 
separation was sufficiently great a one to allow of some intervening alter- 
ation is suggested by the occasional presence of pebbles in the Keweenawan 
conglomerates which may be regarded as derived from the Huronian. On 
Thunder Bay the Keweenawan sandstones carry chert and jasper pebbles 
from the underlying Animikie Group, and on the Montreal River, in 
1 Owen’s Geological Survey of Iowa, Wisconsin, and Minnesota, p. 408 ct seq. 
