ROCKS OF HURON AND SUPERIOR IDENTIFIED. 95 



isfactory and indisputably conclusive. * * * Successive formations of the lowest 

 fossiliferous group of North America were each, in one place or the other, found in 

 exposures divested of all vegetation, resting in unconformable repose in a nearly 

 horizontal position upon the tilted beds and undulating surface of the quartz rock and 

 its accompanying strata, filling up valleys, overtopping mountains, and concealing 

 every vestige of dikes and copper veins. * ^ ^ The chief difference in the copper- 

 bearing rocks of lakes Huron and Superior seems to lie in the great amount of amyg- 

 daloidal trap prese-nt among the latter and of white quartz or sandstone among the 

 former. But on the Canadian side of Lake Superior there are some considerable areas 

 in which important masses of interstratified greenstone exist without amygdaloid, 

 while white sandstones are present in others, as on the south side of Thunder bay, 

 though not in the same state of vitrification as those of Huron. But notwithstanding 

 these differences, there are such strong points of resemblance in the interstratification 

 of igneous rocks and the general mineralized condition of the whole as to render their 

 positive or proximate equivalence highly probable, if not almost certain ; and the 

 conclusive evidence given of the age of the Huron would thus appear to settle that of 

 the Lake Superior rocks, in the position given them by Dr. Houghton, the late state 

 geologist of Michigan, as beneath the lowest known fossiliferous deposits." 



Remarks on the Identification. — Oo the foregoing extract the following 

 observations should be made : 



1. The whole mass of strata from the gneiss up to the Potsdam sandstone 

 is conceived as a unit and generalized from as a unit; while in that interval 

 lie two systems unconformable with each other, as already pointed out by 

 Director Logan himself. 



2. This assemblage is distinctly next inferior to the Potsdam sandstone. 



3. The identification of the copper-bearing rocks of the south shore of 

 Lake Superior with those of Lake Huron is erroneous, because : 



(a) The Superior cupriferous rocks have little analogy with those of Lake 

 Huron, the fragraental strata of Superior being porous sandstones and loose 

 conglomerates of felsitic pebbles, while those of Huron are clean vitreous or 

 vitreo-granular quartzites, with firmly imbedded quartzose and jaspery peb- 

 bles ; also the volcanic beds of the Superior series are interbedded aniygda- 

 loids and hold native copper, while those of Huron are crystalline or altered 

 diabases, and the copper consists of sulphides in veins of quartz. 



ih) The real anah)gue of the cupriferous series of the south shore of Lake 

 Superior, as uow^ recognized, is separated from the system which rests on 

 the gneiss by a series of " blackish argillaceous slates and slate conglomer- 

 ates," designated (4) and (5) in Logan's report of 1846-47, and which are 

 the real analogue of the cuprif-erous rocks of Lake Huron. 



(c) The sandstones of Thunder bay, instead of being in the position of 

 the white quartzite of Lake Huron, are in a higher horizon — either the 

 Kewenawan or the Potsdam sandstone. 



4. Logan's opinion of the Cambrian age of the cupriferous rocks of Lake 

 Huron, made by the identifications, held the Cupriferous series of the south 



