1868.] MACFARLANE — GEOLOGY OF LAKE SUPERIOR. 245 



or ' shot copper' of the amygdules, these roeks often contain 

 huge masses of metallic copper, with which small quantities of 

 native silver are associated. Large irregular patches and veins 

 of calcspar, and smaller masses of epidote are frequently met with 

 in these tufaceous melaphyres. 



Porphyrite. - The transitions, which are frequently observable 

 on the south side of Michipicoten Island, from compact melaphyre 

 to porphyrite have been referred to above. Undoubted porphyrite 

 is to be found at the south-west corner of the Island. It possesses 

 a fine-grained greenish red matrix, containing small flesh-coloured 

 crystals of felspar, some of which have striated cleavage planes. 

 The specific gravity of the rock is 2.619, and the matrix is 

 fusible at the edges. In the upper part of the bed the matrix of 

 the rock becomes coarser grained, shewing distinctly felspar and 

 a darker coloured mineral as constituents, with the small felspathic 

 crystals still scattered through it. The felspar predominates in 

 the matrix and determines the colour of the rock, which is dark 

 red. Its specific gravity is 2.626, and it is fusible, although not 

 readily, before the blow-pipe. It separates into blocks, with very 

 decided divisional planes, but of no regular form. Similar rocks 

 are found at the south-east corner of the Island, where also rocks 

 resembling pitchstone and pitchstone porphyry are extensively 

 developed. The black shining impalpable trap, which has the 

 appearance of pitchstone, has a specific gravity of 2.573. It is 

 fusible to a brown glass, and sometimes contains small colourless 

 felspar crystals. Where these accumulate, there results the rock 

 resembling pitchstone porphyry. The crystals in this rock are 

 frequently recognisable as triclinic. The matrix is fusible to a 

 brown blebby glass, and the specific gravity of the rock as a whole 

 is 2.631 to 2.678. Since the specific gravity of the rock in which 

 no crystals occur is lower than that usually ascribed to melaphyre, 

 and since it is greater than that of true pitchstone, it would 

 appear reasonable to class both these rocks with the porphyrites, 

 or with these porphyries which contain no quartz, to which they 

 probably bear the same relation as true pitchstones bear to felsitic 

 or quartzose porphyries. 



Melaphyre Breccia. — Among the newest of the beds of compact 

 melaphyre, developed on Michipicoten Island, there are sometimes 

 observable beds of a breccia consisting of fragments of dark 

 brown melaphyre, cemented together by a reddish-brown trappean 

 sand. Occasionally the fragments appear rounded, and present 



