On the Norian or " Upper Laurentian" Formation. 189 



if the rock were completely schillerized these products 

 might be quite evenly distributed in it. It may be here 

 mentioned that only in a few places in this Morin area does 

 the plagioclase exhibit that play of colours which is pro- 

 duced by these inclusions in the feldspar from Labrador 

 and elsewhere. 



The plagioclase is almost invarably excellently twinned, 

 according to both the Albite and Pericline laws, the two sets 

 of twin lamellae crossing one another at right angles in 

 the thin sections. This twinning is apparently sometimes 

 secondary and produced by pressure, as for instance when 

 the lamellae appear along a certain line or crack, or when 

 they appear in places where the plagioclase individual is 

 twisted. 



In most cases, however, they are of prmary origin. 

 Frequently in the sections there are a few untwinned 

 individuals of plagioclase which are probably cut parallel 

 to oo P oo (010.) But in certain hand-specimens there 

 is a considerable percentage of untwinned feldspar, resem- 

 bling in all other respects the plagioclase which shows 

 a well defined twin structure. In order to determine 

 whether in these cases two feldspars were really pre- 

 sent, separations by means of heavy solutions were made 

 on material from three hand-specimens from different 

 localities in the thin sections of which these untwinned 

 feldspars occurred, in considerable quantity. Since, how- 

 ever, in a solution having a specific gravity of 2.6*7 all 

 the constituents sank, these untwinned individuals can- 

 not be more acid than labradorite, to which variety the 

 remaining feldspars likewise belong. Similar occurrences 

 of untwinned plagioclase have been often observed. 

 Hawes 1 who investigated some of them, gives an analysis 2 

 of an ordinary specimen of typical labradorite of St. 

 Paul's Island and adds : " Some of the anorthosites de- 

 scribed by T. Sterry Hunt in the Geology of Canada, 1863, 

 were proved by his analysis to be composed of pure labra- 



1 Hawes, On the determination of feldspar in thin sections of Rocks, Proc. 

 Nat. Mus.. Washington, 1881, p. 134. 



2 See table of analyses at conclusion of paper. 



