42 THE CANADIAN NATURALIST. [March 



Insoluble part— 



46.69 



Silica 3823 



Alumina 9 ' 65 



Ferric Oxide 2.83 



Lime 057 



Magnesia 0-33 



Alkalis (by difference) 0-99 52 • 60 



99-29 



From this analysis it would appear that almost the whole of 

 the hornblende has been converted into a chloritic mineral. The 

 insoluble, probably, consists of the felspathic constituent mixed 

 with the quartz contained in the rock. The small rocky island 

 called Pyritic Island, lying to the north of Great Shaginah 

 Island, consists of diorite. In the centre, and running along its 

 length, is a band of the same rock about thirty feet wide, more or 

 less impregnated with copper, magnetic and iron pyrites. 



CoRSlTE. — By far the greater number of the rocks forming the 

 dykes consist of a small-grained mixture of glittering hornblende 

 in large quantity, with felspar, which, being easily decomposable 

 by acids, is probably anorthite. The specific gravity of these 

 rocks varies from 2*934 to 3-085. They sometimes shew a 

 warty appearance on the surface, especially when much weathered, 

 and, occasionally, they are found to break up entirely into small 

 friable pebbles. Islet No. 5, although it seems to be part of the 

 same intrusive mass as Silver Islet, consists of a rock with the 

 mineralogical composition of corsite. It is small-grained and 

 crystalline, shewing abundance of small glittering faces, belonging 

 to its black-coloured constituent, which preponderates over the 

 lighter-coloured felspathic grains. Its specific gravity is 2-916 to 

 2-933. Its powder has a slate-grey colour, which, on ignition, 

 changes to dark brown. On digestion with hydrochloric acid, 

 and afterward with dilute potash ley, it leaves 43-64 per cent, 

 insoluble matter of a dark grey colour, and having a specific 

 gravity of 2-955. The following is an analysis of the rock, the 

 compositions of the soluble and of the insoluble portions having 

 been separately ascertained: — 



46.69 



Soluble Part— 



Silica 21-77 



Alumina 1 1 - 6g 



Ferrous Oxide 13 '5° 



Lime 3 99 



Magnesia 1-75 52-70 



