28 THE ROYAL SOCIETY OF CANADA 



Complementary Varieties 



General. — A feature of interest in connection with the norite 

 intrusives is the occurrence of complementary rocks. In the differenti- 

 ation of a magma two types of dykes are commonly formed. In one 

 of these the oxides of magnesium, iron and some of the calcium, with 

 a low amount of silica, are concentrated, and in the second type, the 

 remainder, or acidic portion of the magma is collected. In the case 

 of a granite magma, the first class gives rise to the lamprophyre dykes, 

 and in the second type, we find the aplites, consisting of quartz and 

 feldspar. These dark and light-coloured dyke varieties are spoken 

 of respectively as melanocratic and leucocratic types. Both these 

 types are represented in the intrusives of lake Athabaska. 



Leucocratic Type 



Beerbachite. — The light-coloured aplitic variety of the norite is a 

 beerbachite. It consists predominantly of labradorite with minor 

 quantities of pyroxene and iron ore. It is a fine-grained to dense 

 rock, dull in appearance, and in places dark in colour. Two sections 

 of the rock were studied, one from a dyke near the Carp river, and one 

 from a light-coloured phase of the norite near the mouth of the Stone 

 river. 



Under the microscope the rock is seen to consist of labradorite, 

 monoclinic and orthorhombic pyroxenes, secondary hornblende and 

 iron ore. The iron ore is largely magnetite, but some pyrite occurs. 

 The magnetite is in the form of irregular grains and long, narrow 

 fragmentai forms. The hornblende is in very small quantities, and 

 is secondary after pyroxene. The monoclinic pyroxene is slightly less 

 abundant than the orthorhombic variety, and has the properties of 

 diallage. The orthorhombic pyroxene is a slightly pleochroic hyper- 

 sthene. The dark minerals occur in small quantities scattered irregu- 

 larly throughout the rock. The structure of the rock is saccharoidal 

 consisting of a mass of interlocking grains of approximately uniform 

 size, the panidiomorphic structure of Rosenbusch. The relative 

 proportions of the minerals present, from determinations by the 

 Rosiwal method, are as follow: — 



Magnetite 5 per cent. 



Labradorite 65 " 



Hypersthene 15 " 



Diallage 12 



Hornblende. .. 3 " 



100 per cent. 



