28 The Ottawa Naturalist. 



and a small sample from a dyke near Gunflint Lake north-west of Lake 

 Superior. The phenocrysts of felspar in the Silver Islet specimen, 

 according to Professor Winchell (i) are distinctly angular and not 

 greenish, but greyish in colour. Under the microscope, these felspar 

 phenocrysts are seen to be a plagioclase towards the basic end of the 

 series (very probably labradorite) which has undergone only incipient 

 alteration, whereas, in general the Huronite shows very great decom- 

 position. 



The writer has seen numerous boulders of diabase containing this 

 mineral in the region to the north and northeast of Lake Huron, 

 especially on the shores of Lake Huron from Killarney westward to the 

 mouth of the Spanish River, 



During the summer of 1893, the writer also noticed a boulder of 

 dark green diabase, on the west shore of Bear Island on Lake Temagami, 

 with plagioclase phenocrysts, which bore a very marked resemblance to 

 the more altered Huronite. As the felspar seemed so fresh and glassy 

 in places, it was thought an optical examination accompanied by a 

 chemical analysis would throw a great deal of light on the original 

 character and composition of Huronite. Dr. Harrington kindly under- 

 took the analysis of this felspar, which proves it to be labradorite' 

 Under the microscope most of these crystals are quite fresh, although 

 certain portions are more or less clouded by the presence of 

 decomposition products, which it is often difficult to resolve, even with 

 the higher powers of the microscope. Certain of the crystals, however, 

 show the same alteration, only in a lesser degree, as that which charac- 

 terizes the Huronite. 



It will thus be seen that the mineral is by no means so rare as some 

 have supposed, but has, on the contrary, a wide geographical distribu- 

 tion. The sole reason of its not being discovered, "in situ," earlier 

 seems to have been due to the necessarily hurried and imperfect 

 explorations first undertaken through these wild and unsettled districts. 

 In 1885, Dr. B. J. Harrington, of McGill University, Montreal, 

 decided to undertake an examination of the Pogamasing mineral for 

 purposes of comparison with that contained in the original Drummond 



(1) No. 601, 10th Annua] Report, Geological Survey, Minnesota, p. 56. 



