On the Norian or "Upper Laurentian " Formation. 299 



forms similar zones around the garnet of some periclotites 

 consists likewise of a mixture of spinel and amphibole. 



The olivine and the minerals which form the zones 

 around it are quite differently orientated; the width of the 

 zones, as we observe them in the thin sections, has no 

 definite relation to the size of the grains of olivine, espe- 

 cially as it varies greatly with the direction in which the 

 crystal is cut. The zones have apparently originated from 

 the interaction of the molecules of silicate of lime of the 

 plagioclase and of the basic silicate of magnesia and iron 

 of the olivine, giving rise to silicates of intermediate 

 composition, that next to the olivine being a more acid 

 silicate of magnesia and iron, which is followed, nearer to 

 the plagioclase, by an acid silicate of lime and magnesia. 

 The edges of the original grains of olivine are evidently 

 the sharp lines which separate the rhombic pyroxene from 

 the hornblende, and the latter undoubtedly penetrates 

 the plagioclase. On the other hand one can often observe 

 the augite starting from this line and growing into the 

 olivine, especially wdiere the olivine remaining has the 

 form of a narrow wedge-like grain which runs out into a 

 line, on either side of which can be found the pyroxene 

 individuals. 



The opinion has been expressed that these zones were 

 produced by dynamic forces which have acted upon the 

 rock. This may be so elsewhere, but here there are no 

 facts which favor this view. 



They are well developed, even in places where the rock, 

 as above mentioned, is quite massive, and there are no 

 facts observed which point to dynamic action. They are 

 found just as well developed in other parts of the anortho- 

 site area, which likewise show no trace of dynamic action. 

 They certainly occur in some localities in the district 

 under consideration accompanied by a cataclastic struc- 

 ture, but this must necessarily be the case if the zones 

 existed before the development of the structure. A single 



