= 
306 8J. D. Whitney on the Huronian and Laurentian 
the northwest, from which the azoic rocks are easily distinguished 
by marked differences of lithological character, and by uncon- 
formability of position. ; 
r. Logan, however, recognizes two distinct systems below 
the Potsdam sandstone, and to which he has given the names of 
Huronian and Laurentian, thus differing from Mr. Foster and ie: 
myself, who have admitted the existence of only one. Ifweex- 
et to its western extremity. We must also admit that these — 
- eupriferous rocks are identical in age with the series of quartz, 
beds, and jaspery conglomerates displayed on the north shore of 
Lake Huron, and hence called “Huronian.” Therefore, according 
to Mr. Logan’s views, since the cupriferous series of Lake Supe- 
Tior rests unconformably on a still lower formation of slates, 
d. 
principal question to be settled, then, is this; what are 
+ the relations of the cupriferous rocks of Lake Superior ?—do 
z® “— constitute a distinct system by themselves, or are they pa 
a, Pa 
local 
& 
poate not to be removed from the place to which they belong, 
a $ 
“a y Mr. Logan as Laurentian. 
£ The sandstone of the northwest, the base of the fossiliferous 
_ Series, which is, as usually seen, three to four hundred feet in 
_ thickness, is made up of a coarse grained and rather friable 
grit, 
