of Lake Superior, 253 



daloid frequently loses them altogether; becoming insensibly a 

 homogeneous amphibolic rock, having a trifling portion of feldspar, 

 now and then barely visible. A lofty isle, fifty miles east of Thunder 

 Headland, in the Pay Plat, displays a cliff of this kind fissiy[ed 

 vertically ; and defended at its base by a shapeless deposit of 

 one of the conglomerates described above. I cannot determine 

 the mutual relations of these rocks. The Grange, and many 

 low isles in the Mammelles, are of this trap. The amygdaloid 

 of these regions may be stratified, but I could not detect 

 the fact. When free from foreign matters, it is massive ; when 

 full of them, it is decayed and broken into accidental forms by 

 the waves. 



From one mile east of the Detroit, on the outside of the island 

 of St. Ignatius, to rather more than two miles west of Gravel Point, 

 a distance of twenty miles and a half, porphyries prevail along the 

 Canoe Route. Like the other rocks of Lake Superior, this por- 

 phyry is found at all levels, its rude colonnades plunging into the 

 waters and crowning the highest summits. Having never seen 

 this rock in contact with any other, and having never observed a 

 decided stratification, I can only speak doubtfully of its relation to 

 the formations around it ; but the following facts bespeak a certain 

 connexion : — Major Delafield and myself, separately, have noticed 

 bowlders of well-characterized porphyry, which contain the rounded 

 masses of calcspar, common in amygdaloid, and coated with 

 green earth ; a substance, abundant here, disseminated, and co- 

 louring the rock. The calcedonies, fluor, veined and fortification 

 agates also of the amygdaloid, are frequent in the porpyhry ; except 

 the second, which is only occasional. Its base is argillaceous 

 chiefly, and when exposed to weather becomes pale reddish white 

 and powdery. It can then scarcely be distinguished from a species 

 from Arran. I have strong reason to believe that this rock passes 

 into the sandstone of St. Ignatius, S;c. Considerable tracts of it 

 exist on the south side of this island, wholly deprived of its feld- 

 spar crystals, and having only here and there a small fragment of 

 limpid quarti! in a more or less compact red cement: and great 

 portions there below water-mark lose even these vestiges of their 



