666 



NORMAN L. BOWEN 



THE LOST LAKE SILL 



The best exposure of granophyre found lies along the west shore 

 of Lost Lake (Fig. 4). In the hand specimen it would be termed a 

 hornblende syenite. It lies at the top of a diabase sill which diamond 

 drilling has shown to have a thickness of more than 500 feet, probably 

 much more. Here a vertical thickness of about 30 feet of granophyre 

 is exposed on the hillside overlooking the lake. Capping the hill is 

 a thin veneer of nearly fiat-lying sediments. It is difficult to draw an 

 exact line of contact. Between what is unquestionably altered sedi- 

 ment, and granophyre is a layer about i foot thick of a purely feld- 

 spathic rock, red in color, and very similar to the granophyre. The 

 microscope shows only feldspar (albite), with a little calcite and 

 blotches of chlorite. The altered sediment close to this feldspathic 



Fig. 4. — Ideal section at Lost Lake, 

 di. = diabase; gp. = granophyre. 



sch. = Keewatin greenstone; si. =slate; 



.layer has the granular appearance of a fine-grained indurated red 

 arkose and under the microscope shows a mosaic of quartz and 

 feldspar, some of which is determinable as albite. This gradually 

 assumes a deep purplish-red color and in places passes into apparently 

 unaltered slates. The whole change takes place within a distance 

 of about twenty feet. 



Going northward from the exposure just described there is a rather 

 quick passage from granophyre, through diabase rich in granophyric 

 material, to normal diabase, which is itself found in contact with little- 

 altered slates. The granophyre is, then, not everywhere present at 

 the upper contact of this sill, but for some reason is localized. 



The granophyre itself is very similar to the Lily Lake rock. 

 Albite phenocrysts are a little less abundant. A small part of the 

 feldspar of the micropegmatite is microcline with, as before, some 

 microperthite. The chlorite occurs in long blades probably secondary 

 after hornblende. Calcite, apatite, and iron ores are again present. 



