1868.] MACFARLANE — GEOLOGY OF LAKE SUPERIOR. 249 



surrounded by gnessiod rocks. Furthermore, the lower members 

 of the Mamainse series are intersected by numerous dykes, con- 

 sisting; of compact melaphyre. In some of them, the constituents 

 of that rock are distinguishable, but most of them are almost 

 impalpable, vary from a reddish-brown to a dark green colour, and 

 frequently exhibit at their sides bauds of slightly different colours, 

 which run parallel with the side-walls of the dyke. 



The average strike of the Upper Copper-bearing rocks of Michi- 

 picoten Island is N. 68° E.. and the dip 25° south-eastward. An 

 approximative estimate of their thickness is as follows: — 

 ( h-anular, delessitic and compact melaphyres, 



and conglomerates 10.000 feet. 



Compact melaphyres with agate amygdules. 4,500 " 

 Resinous traps, porphyrites and breccias. . . 4,000 " 



18,500 feet. 

 If we compare the rocks of Michipicoten Island with those of 

 Mamainse, it would appear that the inferior rocks of the latter 

 group do not come to the surface at Michipicoten Island, and that 

 the higher rocks of the Michipicoten group have not been de- 

 veloped at Mamainse, or lie beneath the waters of the lake to the 

 south-west of the promontory. It would therefore appear just, in 

 estimating the thickness of the Upper Copper-bearing rocks of the 

 eastern part of Lake Superior, to add to the Mamainse series the 

 above mentioned 4000 feet of resinous traps or porphyrites, which 

 would make the whole thickness at least 20,000 feet. The rocks 

 of the west and south shores of Michipicoten Island present the 

 most regular appearance, and it might be expected that those of 

 the south shore would, from their strike and dip, repeat them- 

 selves on the east side. But, as in the case of Mamainse, such an 

 expectation is disappointed. On examining the rocks of the east 

 shore, the upper beds, consisting of the porphyrites above men- 

 tioned, seem regular enough, but beneath these come brecciated 

 melaphyre, delessitic melaphyre cut by a porphyritic rock, and 

 others in which the evidences of bedding are very indistinct. 

 Among these rocks the two following may be particularised as 

 occuring in large masses. The first has an impalpable flesh-red 

 or reddish-grey matrix, wherein occur numerous grains of dark 

 grey quartz, and also light-coloured soft particles, which 

 seem liable to removal by atmospheric agencies, giving 

 the rock where this has taken place a porous appearance. 



