1869.J MACFARLANE — GEOLOGY OF LAKE SUPERIOR. 43 



52-70 

 Insoluble Part — 



Silica 24 '39 



Alumina 3 • 78 



Ferrous Oxide 11 "38 



Lime 3 '42 



Magnesia 0-67 43-64 



Water 315 315 



99-49 



The composition of the insoluble portion, calculated to 100 

 parts, is as follows : — 



Silica 55-88 



Ferrous Oxide 2609 



Alumina 8'66 



Lime 7 ' 84 



Magnesia 1-53 



100 '00 



Judging from these figures, and the appearance of the black 

 constituent in the rock itself, it would appear reasonable to regard 

 it as basaltic hornblende. The large quantity of mineral present, 

 decomposable by acid, would lead to the inference that the 

 felspathic constituent is anorthite, although, doubtless, some 

 chloritic substance is decomposed and dissolved with it. The 

 presence of anorthite in these fine-grained rocks is confirmed by 

 its occurring in some of them in well-developed crystals, constituting 

 the rock which is referred to on page 72 of the Geology of 

 Canada, and which is next de^ciibed. 



Anoethite porphyry. — The dyke which forms the rocky 

 islets, marked 1, 2, and 3 on the map, and which runs along the 

 south-east side of Burnt Island, consists of this rock, although 

 the anorthite crystals are but sparingly distributed. The most 

 characteristic development of this porphyry occurs on the shore 

 between Location Bay and Perry's Bay, constitutes Anorthite 

 Islet, and then joins the mainland on the east side of Perry's 

 Bay. The size of the crystals varies from one-quarter-inch to 

 several inches in diameter ; they are beautifully striated, and 

 aggregations of them, two or three feet in diameter, are of 

 frequent occurrence on Anorthite Islet. Indeed, at a distance, 

 the rock of this islet resembles a breccia, so great is the number 

 and size of the masses of anorthite. These masses seem to have 

 been formed by the crowding together of numbers of anorthite 

 crystals, and some of the spaces between these seem to have been 

 subsequently filled up by quartz. The specific gravity of the 

 mineral from these masses is 2-737. It was analysed by digestion 



