of Lake Superior. '251 



nor so plentiful; the greater part of its rock is bluish black, fine 

 granular, or compact. The strongest cleavages pass N.N.E., but 

 none are so marked and continuous as to point out a stratification. 

 I had it not in my power to examine minutely the mineralogical 

 characters of Thunder Mountain; but the native debris strewn along 

 its base, its general features, and the form and weathering of its pal- 

 lisades (as a similarbutfar less splendid structure on the River Hud- 

 son, New York, has been named) are evidences of its complete iden- 

 tity with the rocks of the coast from hence to the Grand Portage, 

 and in longitude 90° forming the height of land between the waters 

 of Lake Superior and Hudson's Bay. It is a greenstone trap passing 

 from sienitic to homogeneous ; and I think gradually. The most 

 common forms are, moderately fine granular and small sienitic ; 

 the feldspar, in the latter case, appearing in white, or brownish 

 gray rhomboidal facets, or in grains. The colour is dark blackish 

 brown, bluish black, and even red, from the presence of iron. The 

 fracture is conchoidal when massive — when schistose, the cross 

 fracture is uneven or earthy. The fine grained and compact 

 greenstone traps arc usually (but not always) in the lower parts 

 of the cliff, in horizontal layers, eighteen inches broad, and dimi- 

 nishing to the state of shale. The upper parts again, are often 

 crystalline and almost always fissured perpendicularly, from one 

 foot to ten or fifteen in breadth, and of great length. They are 

 rendered prominent and conspicuous by the truncation of their 

 lateral angles. The transition from the vertical to the horizontal 

 fissure is effected in the following manner. In the variable in- 

 terval between these two appearances in their perfect form, the 

 place of the pilastres is shewn by perpendicular fissures ; 

 some of these gradually vanish, while others, being prolonged, 

 curve at their lower ends rather sharply upwards, and then close ; 

 succeeded below by short and interrupted rents, waving, or rather 

 discoid, and always more or less horizontal. These soon become 

 (downwards) straight, and finally, continuous lines. One of the 

 Welcome Isles in Thunder Bay affords a good instance of what is 

 here attempted to be described. It is by no means singular to 

 have the vertical and horizontal fissures equally strong, or nearly 



