Publisher's Synopsis
Excerpt from Notes on a Few Dykes Cutting Laurentian Rocks, More Especially With Reference to Their Microscopic Structure
III. River St. Simon. (plate. Fig. 2. This specimen is from a tine-grained, greyish-black dyke which cuts the Great. Lake band of crystalline limestone on the St. Simm, a small tributary of the North River, in Terrebonne County cast of Grenville. I am indebted for it to the Director of the Geologi cal Survey. The dyke probably belongs to the same set as the Grenville ones just described, its general structure being the same, but it has apparently undergone very little alteration the section being beautifully clear and transparent. With the micro. Scope it is seen to'con'sist. Of a network of plagioclase feldspar, with augite, magnetite and apatite C?) and a very little viridite. The feldspar as seen in the section is perfectly transparent and colourless, and with the polariscope shows a beautifully banded structure In places it contains microlites which are possibly apatite, and also a few vapour or gas-cavities, generally in groups. The augite is pale greyish-brown frequently penetrated by blades of feldspar and often containing groups of minute grains of magnetite. It appears to constitute about half the rock. The magnetite occurs mostly in irregular grains and masses of most fantastic sh-'pe. But now and then in rude crystals and red like forms. In some cases it is seen to be penetrated by blades of feldspar. (sec figures on next page.)
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