228 On the Geography and Geology 



" with respect to mathematical reasoning in general, when it is 

 cautiously applied, it has enabled us to arrive at physiological 

 truths, which we perhaps could not have attained by any other 

 method, and which are beyond the reach of actual observation." 



Art. III. Notes on the Geography and Geology of Lake 

 Superior. By John J. Bigsby, M.D., F.L.S., and M.G.S. 



[Concluded from p. 34.] 



I HAVE now to describe with some minuteness, the nature, con- 

 tents, and connexion of the rocks above enumerated, as they occur 

 in succession on the north shore ; and commencing from Gros Cap. 

 This concluded, I shall give a rapid view of the principal points in 

 the geology of the south shore. 



The south headland consists of a very compact, brick-red por- 

 phyry, which extends a mile northward in broken scarps, and in 

 perpendicularly fissured clifFs. It is the same as certain varieties 

 of the porphyries of the Pay Plat, excepting that it is not quite so 

 slaty, and that the colour is paler. Its paste is homogeneous and 

 fine grained ; the fracture, obtained with difficulty from its facility 

 of division along the natural cleavages, is imperfectly con- 

 choidal, passing into uneven ; the lustre is dull, — in hardness, it 

 scarcely yields to the knife. The odour of clay is very strong on 

 being moistened. It contains numerous small irregular masses of 

 limpid quartz and small crystals of pale-red feldspar. There are 

 druses of pyramidal and rhomboidal calcspar ; and also epidote, 

 in fissures, and in druses of capillary crystals. It is full of con- 

 fused rents, large and smooth, but I could perceive no strati- 

 fication. 



This porphyry is replaced suddenly, without any change in 

 the aspect of the hills for a second mile, by greenstone, blackish 

 blue, fine granular, and small crystalline, with occasional small 

 crystals of red feldspar, according to the specimens I took from 

 the isle 1000 yards north of the porphyry. I there observed, as I 

 thought, traces of stratification, for three or four yards in parallel 



