1869.] MACFARLANE — GEOLOGY OP LAKE SUPERIOR. 41 



high angles, the dip in the latter case being generally to the 

 south-east, but sometimes also to the north-west. They vary in 

 thickness from a few feet to nearly a hundred, and they sometimes 

 exhibit interesting phenomena as to joints of separation. Irre- 

 gular columnar separation at right angles to the inelination is 

 frequently observed, but it is on a ponderous scale, and although 

 it reminds one of trappean jointing, it is not at all so regular. 

 Sometimes the dykes are split up into large square blocks, or into 

 large flat pieces, with their planes parallel to the sides of the 

 dyke. Although the direction above given for these dykes is the 

 prevailing one, it will be seen from the map that some of them 

 have courses more or less divergent from this strike; indeed, 

 some small dykes are to be seen branching off from the main ones. 

 The rocks which constitute these dykes belong to the diorite 

 family, but are capable of being subdivided into several species, 

 according to the nature of the felspar they contain. The recog- 

 nition of their constituent minerals is a matter of some difficulty, 

 as they are, for the most part, small, or fine-grained. No instance 

 was observed of a coarse or large-grained rock among these dykes, 

 although a very distinct porphyritic rock was met with. As 

 examples of the various species of rocks constituting these dykes, 

 the three following may be particularized. 



Diorite. — The nearest approach to this species is the rock 

 constituting Silver Islet. It is distinctly composed of a greenish 

 black and a white mineral, the former being, however, duller in 

 lustre and less hard than the hornblendic constituent of the rocks 

 of many of the other dykes. Quartz is occasionally detected, 

 copper and iron pyrites and a grain or two of schiller spar, also. 

 Its specific gravity is 2-713 to 2-711. Its powder is greenish grey, 

 changing on ignition to leather- brown, and yielding water. On 

 digestion with hydrochloric acid, and then with weak potash ley, 

 52-6 per cent, of a residue is left, which is almost pure white in 

 colour. The following is an analysis of the rock, shewing the 

 composition both of the soluble and insoluble part: — 



Water 5-02 



Soluble part— 



Silica 15" II 



Alumina 582 



Ferrous Oxide 

 Lime 



Magnesia 1-50 41-67 



46.69 



