On Some Dykes Containing Huronite. 41 



<( 



train shadows " due to pressure. A good proportion showed an in- 

 cident uralitization. The plagioclase of the groundmass has also 

 undergone more or less " saussuritization " and occurs in stout and 

 rounded laths thus producing a rather coarse ophitic structure. The 

 ilmenite present in irregular grains is often fresh but shows occasional 

 incipient alteration to leucoxene. A small amount of chlorite is also 

 present. 



7. Locality Shore of Drummond Island, Lake Huron, (from a 

 boulder.) 



The slide was made from a fragment, obtained through the kind- 

 ness of Dr. Harrington, from a duplicate specimen of the original 

 boulder at present in the Holmes collection of the Peter Redpath Mu- 

 seum of McGill University. The first examination and analysis by Dr. 

 Thomson was rather imperfect as pointed out by Dr. Harrington (1) 

 but it has been thought advisable to reproduce the analysis, though im- 

 perfect, for purposes of rough comparison. This analysis is as follows : 



Silica 45 ' 80 



Alumina 33 92 



Ferrous Oxide 432 



Lime 804 



Magnesia 172 



Loss on ignition 4. 16 



97-96 



The specific gravity, according to Dr. Thomson, is 2-8625. Under 

 the microscope the phenocrysts of the so-called " Huronite " are seen to 

 be a decomposed aggregate of zoisite, muscovite, epidote, calcite, chlo- 

 rite and felspar. Occasionally there is a very narrow border of compa 

 ratively unaltered felspar surrounding these individuals, in which traces of 

 the very fine striation, due to multiple twinning, may be observed. Im- 

 mediately within this band, however, the decomposition products are most 

 abundant, and the original plagioclase is replaced almost altogether by 

 epidote, zoisite and muscovite, their relative abundance being in the 

 order mentioned, while the interior of the crystals is composed mainly 

 of muscovite with a much less proportion of zoisite, epidote and felspar. 



(1) Trans. Royal Society of Canada, Section III., 18S6, p. 82. 



