74 Geological Society. 



they are composed of quartzite, mica-schist, phyllite, slate, volcanic 

 breccia, and greywacke. 



Far more interesting are the nickel-bearing rocks, which are 

 eruptive and form long elliptical stocks which conform to the strike 

 of the Huronian rocks containing them. Contact-action indicates 

 that they are younger than the rocks previously referred to. The 

 smaller eruptives are composed of greenstone, which appears to have 

 been formed from norite or gabbro. Some of the larger eruptives, 

 however, have been highly differentiated on cooling, as they are now 

 composed of granite and greenstone with gradual transitions from 

 the one to the other. The greenstone generally forms one side of 

 the eruptive, and on the outer border is often characterized by 

 large masses of nickeliferous pyrrhotite, chalcopyrite, and nickeli- 

 ferous pyrite, with frequent smaller masses of magnetic iron- ore 

 rich in titanic acid. The writer regards these mineral masses 

 as genetically related to the greenstone and granite, in that they 

 appear to be the extreme products of differentiation. About half 

 the world's nickel supply is drawn from these deposits. 



The greenstone is generally somewhat altered, but at times it ia 

 only slightly changed, when it is seen to be typical norite. 



The alteration of augite and hypersthene is described in detail, 

 and the term ' migration ' is suggested for the process in which 

 secondary hornblende is formed in plagioclase, as if bastite-sub- 

 stance had been carried in solution along the cleavage-lines of the 

 felspar and by reacting upon its host had produced secondary horn- 

 blende. Hornblende so formed is referred to as ' emigrated horn- 

 blende.' Areas of uralitic hornblende generally extinguish under 

 crossed nicols in two portions. On a very favourable section it was 

 possible to determine that the two portions of hornblende are 

 in definite crystallographic orientation — namely, that of twinning 

 on the orthopinacoid, as is common in hornblende-crystals. 



A mineral resembling wohlerite was found to be relatively 

 abundant in some of the more acid rocks of the "Windy Lake 

 eruptive. 



The nickeliferous rocks are cut by younger eruptives — stocks of 

 granite and dykes of olivine-diabase. 



A few pages are devoted to the subject of differentiation, in 

 which it is pointed out that no one of the generally accepted 

 theories is able to account for all the phenomena observed in the 

 differentiation of the nickel-bearing rocks of the Sudbury district. 

 Stress has been laid upon the part which gravitation undoubtedly 

 plays in producing heterogeneity in eruptive rocks. All the old 

 theories of differentiation are directed to explain the presence of 

 basic borders on more acid central portions, while they do not 

 account for those cases where the central portions of stocks are 

 basic and the margin acid. They fail also to give any explanation 

 of the commonest case, namely, the eruptive showing little or no 

 differentiation. These different cases are not only explained, but 

 predicted, by the application of the principle of gravitation to slowly- 

 cooling eruptive magmas. 



