Section IV, 1920 [25] Trans. R.S.C. 



The Norite Rocks of the Lake A thahaska Region 



By F. J. Alcock, B.A., Ph.D. 

 Presented by D. B. Dowling, B.Sc, F.R.S.C. 



(Read May Meeting, 1920) 



Intrusive into the Pre-Cambrian gneissic complex north of Lake 

 Athabaska are numerous dyke rocks consisting of granites, pegmatites 

 and aplites, lamprophyres, amphibolites, gabbros, norites and diabases. 

 From a petrographical point of view the norite rocks are the most 

 interesting and a brief summary concerning them is given below. 



Great Relations 



The norite rocks outcrop towards the eastern end of lake Atha- 

 baska and on both sides of the Stone river which enters the lake from 

 the east. In the country north of the lake it is most frequently found 

 in the form of sill-like intrusions along the foliation planes of the old 

 gneissic complex; but in places it is also found in dykes, cutting across 

 the regional foliation. The extent of the larger intrusions has not 

 been determined. 



General Character 



The rock presents considerable variation in aspect and composi- 

 tion. Freshly broken surfaces are commonly dull and the feldspar 

 and pyroxene constituents show no traces of cleavage planes. A few 

 small bright specks of pyrite may generally be observed. Weathering 

 imparts a pinkish-gray coloration to exposed surfaces and at the 

 same time develops evidences of a slight foliation. 



Even in the coarser grained varieties individual crystals cannot 

 be distinguished in hand specimens;, the rock in this type having the 

 appearance of a mixture of dark and light coloured materials. In the 

 denser, more uniform varieties the rock has a greyish-black colour. 



In thin section the main type is found to consist of plagioclase, 

 hypersthene and iron ore. The plagioclase is labradorite, and much 

 of it is unstriated. Many of the crystals show a combination of peri- 

 cline and albite twinning. Many sections show evidences of granu- 

 lation. Some of the feldspar crystals have been broken into small 

 fragments; hypersthene occurs as large irregular crystals surrounded 

 by broken fragments. Small quantities of magnetite and pyrite and 

 minute shreds of brown biotite occur as accessory minerals. 



