40 The Ottawa Naturalist. 



or in a direction nearly at right angles while the scales and plates ot 

 sericite have a similar development. 



The specific gravity, ascertained by Mr. R. A. A, Johnston, of these 

 porphyritic crystals was 2758. 



The matrix of these crystals is a rather fresh diabase with pro- 

 nounced ophitic structure and composed chiefly of plagioclase an" 

 augite. The plagioclase is idiomorphic and forms an interlacing net- 

 work of lath-shaped crystals. Occasional crystals are rather fresh and 

 glassy, but usually they exhibit the same alteration as the larger por- 

 phyritic individuals, and apparently belong to the same species of felspar 

 (labradorite). The decomposition products aggregate themselves to- 

 ward the centre of the crystal leaving a somewhat fresh periphery. The 

 augite is in general quite fresh, but occasionally an individual was seen 

 partially altered into green, strongly trichroic hornblende. Twins are 

 common. A considerable quantity of biotite is present which is al- 

 ways more or less altered to chlorite, llmenite, an abundant consti- 

 tuent, occurs in irregular grains and only shows incipient alteration to 

 leucoxene. Occasional prisms of apatite were noticed, chiefly developed 

 in the chloritized biotite. The more unaltered portions of the plagio- 

 clase show the undulatory extinction due to pressure. Pynte is also an 

 abundant constituent. 



6. Locality. Algoma Mills, north shore of Lake Huron, district 

 of Algoma, Ontario. (1) 



The thin section exhibits a rock very similar to the one just des- 

 cribed and must be regarded as being derived from a dyke almost anal- 

 agous in character and composition to that exposed near Murphy Lake. 



The phenocrysts of labradorite show the usual alteration into an 

 aggregate composed chiefly of muscovite, epidote and zoisite although 

 considerable portions of some of the crystals are free from these de- 

 composition products. The augite has a light yellowish colour and is 

 only slightly pleochroic. Twins are common, the twinning plane and 

 composition face beinc the orthopinacoid. 



Curved or distorted individuals were often noticed exhibiting the 



"From a boulder No. 1605, Geological Survey of Minnesota, series of rocks, 18th 

 Annual report, [age 58. 



