34: The Ottawa Naturalist. 



the phenocrysts of plagioclase become a dull greyish white, thus ren- 

 dering the rock very conspicuous. In general the rock bears a very close 

 megascopical resemblance to the diabase originally described as con- 

 taining the Huronite while the phenocrysts themselves differ only in the 

 degree of alteration they have undergone. The writer regards this 

 diabase as the least altered representative of the series of rocks studied 

 but which, under similar conditions, would have furnished a rock dif- 

 fering but slightly, if at all, from any of the more decomposed speci- 

 mens first noticed and described as containing " Huronite." 



An analysis of a portion of one of the least altered of these pheno- 

 crysts of plagioclase, kindly undertaken by Dr. Harrington of McGill 

 University, proves the species to be labradorite. The followiug are 

 the results : 



Silica 54' 19 



Alumina 28 ' 42 



Ferric Oxide o 77 



Ferrous Oxide 041 



Manganous Oxide Trace 



Lime 1047 



Magnesia o' 52 



Soda 4-47 



Potash o ' 63 



Loss on ignition 59 



100 47 

 The specific gravity of carefully selected fragments with the bottle 

 was 2 679. 



Under the microscope the rock is seen to be a very typical and 

 rather fresh olivine-diabase. In many instances the large phenocrysts 

 are quite fresh and give the extinction angles characteristic of labra- 

 dorite. Very often, however, irregular areas and patches have under- 

 gone considerable " sericitization," the resulting scales of hydrated mus- 

 covite being very minute. Occasionally this alteration is carried farther 

 and both zoisite and epidoteare present in addition to the sericite as a 

 result of secondary action. At times a narrow border surrounding those 

 crystals exhibits a micro-perthitic structure. A careful examination ad- 

 duced sufficient evidence to indicate clearly that a more extended alter- 



