Latest KvKpflres of Ltikc Sujicrior Ih'fiion. — IViiir/tet/. 271 
"bly for this reason that Irving, who ))iit such amongst his Ke- 
weenawan, in Wisconsin as well as in Minnesota, included 
some of these islands in that formation, although, as now 
known, these felsytes and granites are principally, if not 
wholly, pre-Keweenawan. The northern rim of the great sjni- 
clinal, therefore, is a persistent feature. When the dip of the 
Keweenawan and Animikie appears reversed a different great 
synclinal is expressed. But so far as known, throughout the 
Lake Superior region this dip is not reversed. It is always 
toward the great basin. This structural fact again causes it 
to appear improbable that the horizontal sandstones of Black 
ba}', which lie north of the northern rampart and non-con- 
formabl}^ upon the Animikie, are an integral part of the Ke- 
weenawan. Irving put them ii] the base of the Keweenawan, 
probabh^ because of the great (|uantity of diabase with which 
they are associated, apparently not a})prehending the fact that 
the strike of those beds is further south with a considerable 
dij) to the south, and that it would not be possible for them to 
reverse their dij) within the interval (about IcS miles) without 
manifesting it at some place above the surface of the lake. 
If this inquiry be carried further, it will be necessary, in 
the next place, to examine into the nature of these rocks them- 
selves. 
Dr. Robert Bell has given very full descriptions of them in 
several reports of the Geological Survey of Canada.* Litho- 
logically he regarded them comparable with the Permian or 
Triassic. They consist of light-colored sandstones, often ar- 
gillaceous, sometimes reddish, marls and limestones. When 
the writer examined them in 1S79, on the mainland north from 
Silver Islet he was impressed with the aspect of recentness 
which they present, in contrast with all the other rocks of the 
region. f They lie there in a deeply eroded place in the hori- 
zontal Animikie slates of the region. On Thunder cape the 
Animikie slates rise about 1,()0() feet higher, lying horizontal, 
or having a dip of a few degrees into the lake. In this eroded 
depression these later sandstones and marls lie, also about 
horizontal, non-conformably on tiie Animikie, tlieir base being 
a conglomerate hardly more indurated than the sandstones 
and marls above, and in great contrast with the basal conglom- 
*Report for 18G(5-G9, pp. .'^13-.3G1: ditto 187.3-7-4, p. 97. 
tSee 10th Report, Minnesota Survey. 
