Geology of Lake Superior. 13 
quantity in the Slate Islands, so named by us from that cire 
cumstance, 
These slates are stratified, highly inclined, sometimes 
vertical, much disturbed and shattered, direction nearly east 
and west. Transition limestone has been found by Doctor 
Bigsby only on Thunder Cape, alternating with sandstone. 
13. Trap or overlying rocks. ‘These occupy the same 
extent as those before mentioned, that is, from one extremity 
of the lake to the other. 
The amygdaloid, we have no doubt, rests on the sienitic 
granite, although, from the nature of the country, we never 
saw their junction, It passes into greenstone, simply by 
being divested of its nodules, onthe one hand, and into por- 
phyry, ontheother. At least the porphyry often contains, 
besides its crystals of quartz and felspar, imbedded nodules 
of calcedony, agate, calespar, &c. We traced the amyg- 
daloid from near the eastern extremity of the lake at Point 
Mamainse to Gargantua, thence westward to the south side 
of Michipicoten Island. Itis next found on the Slate Islands; 
and is very abundant, (forming whole islands,) along the 
south side of Neepigon and Black Bays, whence it coutinues 
to Ile Royale, and thence across to Point Keewawonan, where 
its debris is plentiful. We did not see it there in place, but 
we have been told thatit was seen by Mr. Thompson as. 
tronomer of the boundary line commission. It is however 
certainly not the principal formation of that promontory as 
Doctor Bigsby has stated, and who has most likely been mis- 
led by report, or by seeing specimens from the masses on 
the beaches. 
From lle Royale amygdaloid appears at iatervals along 
the north coast south westward all the way to the river St. 
Louis. 
Associated with the amygdaloid are various clay porphy- 
ties 
