246 THE CANADIAN NATURALIST. [Jan. 



more of the character of a conglomerate. Similar rocks are seen 

 in the Point Keweenaw district. 



Porphyritic Conglomerate.- At the sonth-west corner of Michipi- 

 coten Island there is visible a conglomerate bed, the boulders of 

 which consist principally of porphyrite, in which a few minute 

 felspar crystals are discernible. Some of the boulders are granitic, 

 and occasionally pebbles occur consisting of or containing agate. 

 These are enclosed in a matrix consisting of coarse-grained and 

 red-coloured porphyritic or trappean debris. In the upper part 

 of the Mamainse group similar conglomerates are found, but in 

 one instance the matrix seems to consist of the same crystalline 

 materi 1 as the boulders and fragments, and is very firmly cemented 

 to these. The most interesting example of this rock is that of the 

 Albany and Boston mine, near Portage Lake. Here the matrix 

 of coarse-grained porphyritic sand is accompanied by calcspar, and 

 in some places fine metallic copper.* Other porphyritic conglo- 

 merates occur to the south of Portage Lake, some of the boulders 

 of which consist of quartzose porphyry, and the matrix of some 

 of which contains quartz as well as calcspar. 



Felsite-tuff. — Overlying the Albany aud Boston conglomerate a 

 bed of so-called l fluckan ' occurs, which is a fine-grained, dark- 

 reel shaly rock, in which pieces of a greenish blue colour are 

 sometimes seen. Both substances are fusible before the blow-pipe, 

 and contain occasionally small grains and flakes of copper. 



Polygenous Conglomerate. — This name is applied by Naumann 

 and Zirkel to those fragmentary rocks whose boulders consist of 

 two or mare different rocks. Conglomerates of this nature are 

 especially frequent, among the inferior rocks of the Mamainse 

 group, and among those of Keweenaw Point. The boulders of 

 these Mamainse conglomerates are chiefly of granite, gneiss, 

 quartzite, greenstone, and slate, and some of the newer beds con- 

 tain boulders of melaphyre and amygdaloid in abundance. The 

 matrix is generally a dark red sandstone. 



Sandstone. — Among the melaphyres and conglomerates of 

 Mamainse and Point Keweenaw an occasional stratum of sand- 

 stone is found of the same character as that which forms the 

 matrix of the polygenous conglomerates. 



The manner in which the rocks above described are associated 

 with each other, is much more regular than the architecture of 



* This Journal, Yol. hi., Second Series, p. 9. 



