16 The Ottawa Naturalist. 



especially in the smaller individuals they have a rather irregular out- 

 line. The specific gravity of these crystals ascertained by Mr. R. A. A. 

 Johnston was 2935. 



Under the microscope these phenocrysts show a very advanced stage 

 of alteration and the original plagioclase is now replaced by an aggre- 

 gate of muscovite, zoisite, epidote, felspar and calcite. There is little 

 or no trace left of the original twinning lamellae. The plagioclase laths 

 present in the enclosing matrix show a similar alteration, although not. 

 to so large an extent. The augite originally present is now replaced by 

 hornblende (uralite) and often the alteration has proceeded so far that 

 chlorite has resulted. These resulting products of decomposition fill the 

 original allotriomorphic individuals of augite. These individuals as 

 now present usually exhibit a deep gieen border of strongly trichroic 

 hornblende, while the interior is occupied by an aggregate of interlacing 

 fibres of light green hornblende with more or less chlorite. Traces of 

 the characteristic interrupted cleavages of augite are present in occa- 

 sional grains, but no unaltered cores now remain. The resemblence 

 to other uralitic hornblende is, however, unmistakable. ( 1) The hornblende 

 also bears a close resemblence to that present in the rock just described 

 (No. 9) in which cores of the original augite are still present. The 

 ilmenite present is more or less altered to leucoxene showing brilliant 

 polarization colours (compare No. 8 ante). A considerable amount of 

 biotite of a light brown colour on account of the "bleaching" it has 

 undergone shows rather brilliant interference colours. The biotite has 

 also been altered in many cases to chlorite. Granophyre structure was 

 also noticed. 



//. Loca/ilv. Lake Petitsikapow, about 15 miles below old Fort 

 Nascawpee. West branch Hamilton River. Labrador Peninsula. (2) 



The dyke from which the sample was taken, according to Mr. Low, 



is 2co yards in width, coarsely crystalline in the centre where the 



porphyritic individuals of Huronite are often three-fourths of an inch in 



diameter. The dyke breaks through and alters sandstones, limestones 



(i) Williams' Appendix I., Part F. , Annual Report, Geological Survey of 



Vol. V., 1S.S9-90, p. 60. 

 (2) Reference No. 4, p. 3c, Bk. II. Low, 23/6/94. 



