On the Norian or " Upper Laurentian " Formation. 191 



appears in the plagioelase in little spots. It has the optical 

 character of a bastite or pseudophite and the decomposed 

 feidspar resembles therefore to a certain extent that of 

 Waldheim in Saxony described as pyknotrope by Breit- 

 haupt. In another handspecimen of the same rock from 

 New Glasgow the feldspar is changed into a colourless 

 mineral which forms small feather-like clusters. It shows 

 magnificent polarisation colours and has a distinct cleavage 

 to which the extinction is parallel. So far as this rock 

 could be investigated in thin sections, the mineral showed 

 all the optical properties of muscovite. It may possibly be 

 paragonite which cannot be distinguished from muscovite 

 under the microscope, for one would expect a soda mica 

 rather than a muscovite as a product of the alteration of 

 plagioelase. 



Augite. — This constituent is with a few exceptions, gen- 

 erally present in much smaller quantity than the plagio- 

 elase, but is next to it the most abundant constituent. 

 The rhombic pyroxeneis present however in nearly if not 

 quite equal amount. It occurs in irregularly shaped 

 grains of a light green color which are either non- 

 pleochroic or exhibit a scarcely perceptible pleochroism 

 in greenish tints. In sections which are nearly parallel 

 to the base, we see typical cleavages which cut each 

 other almost at right angles and are characteristic of 

 pyroxene. They are often intersected by a third more 

 perfect cleavage which is parallel to ooP oo 000) as 

 shown by its position relative to the plane of the optical 

 axis. In the prismatic zone the mineral shows an extinc- 

 tion angle from 0° to 45°. 



In many sections of the pyroxene there are brownish 

 black tables or small black rods which resemble very much 

 the inclusions of the plagioelase above described. Where 

 these occur they are frequently parallel to oo P 65 (100) ; 

 in other cases instead of being scattered throughout the 

 whole individual they are confined to certain spots. The 

 augite can often be observed to have grown around grains 

 of iron ore. It is generally quite fresh, but in many hand- 



