On Some Dykes Containing Huronite. \~> 



be seen and the specimen doubtless represents the " granitoid " structure 

 so characteristic of the centre portion of most diabase dykes which nearer 

 their margin exhibit the typical ophitic structure. The larger phenocrysts 

 show a marked alteration. Most of the sections of these crystals are 

 made up of innumerable minute scales and fibres of light greenish seri- 

 ate arranged parallel to the polysynthetic twinning lines, and therefore 

 even where the alteration has proceeded farthest the direction of the 

 very fine striation may still be ascertained. Zoisite and epidote have 

 aho been developed the former usually in mure or less elongated 

 prisms or lath-shaped crystals, occurring either isolated or in irregularly 

 disposed groups. The epidote is present in irregular grains or associ- 

 ated with calcite filling certain fissures in the crystals. Some portions 

 of the crystals which had ercaped alteration had a distinctly reddish 

 colour and revealed the fine twinning stria?. The crystals are precisely 

 similar to those decribed by Thompson as " Huronite." The specific 

 gravity of these crystals, according to Mr. R. A. A. Johnston, is 2725 

 The augite has undergone more or less complete uralitization, although 

 in most cases cores of unaltered material remain. During this process 

 a certain amount of epidote present in the slide has been formed. The 

 plagioclase of the matrix shows the same alteration or " saussuritization ' 

 as the larger phenocrysts, the decomposition products aggregating them- 

 selves towards the centre leaving a comparatively fresh periphery. 

 Ilmenite is a rather abundant constituent and occasionally shows 

 incipient alteration to leucoxene. Apatite is very abundant. The 

 interlamination of quartz and felspar, known as granophyre, is present 

 in considerable quantity. 



ro. Locality near Missinaibi Station, on the main line of the 

 Canadian Pacific Railway, District of Algoma, Ont. 



The specimen examined was obtained by Dr. Selwyn from one of 



several dykes which cut both the Huronian and Laurentian rock 



exposed in this region. It is a medium grained dark greyish green 



diabase whose ophitic structure is megascopically apparent. The por- 



phyritic crystals vary from a pale greyish green to a light yellow green, 



weathering to a light grey on exposed surfaces. Very frequently they 



have tolerably good crystallographic boundaries, although in most cases 



