772 A. P. COLEMAN 
The total average was obtained on the basis of two parts basic 
rock to three parts of acid rock, which corresponds roughly to the 
field relations. The ore deposits, really integral parts of the basic 
edge, have been left out in the calculation, since the percentage which 
they form of the whole eruptive is relatively small, though about 
2,500,000 tons of ore have already been mined and many millions 
more are known to exist. Professor Vogt refers to the Sudbury 
gabbro as containing originally about 0.05 per cent. of the ore, which 
may serve as a not improbable guess at the quantitative relations. 
According to the new classification of eruptive rocks, the average 
basic side of the eruptive may be called Harzose, and the acid side 
Adamellose. The norm for the two phases is as follows: 
Basic Acid 
OUENWwoce a kode OM Ob Uomo UeyedocoGut 9.24 20.76 
@xthoclase pete eee irene 8.34 22.24 
PN evi voteaie paceman iA ieee atlaoc car Nearer 19.39 31.06) 
go Nalosadithi Wie ne a Mga ane Oe NO open mier a 258 31:92 6.67 § 37-73 
GHOSSORosunosodoeusmonebelll | obedmagagons 2.44 
Diopside 3) Mig@! {SiO as jer oe ieee cepa crs hell eles edevere ayer “70'¢ 3-00 
FeO! SiOge ae gas Aetdem ate ale SiRUN, Gosia 85 
Wile(@) oS vis-on o00 von o pclbec TiO eet 
Hypersthene } ref) Si@oanee oo eerie Taco Nieuwe 6.34 ole 
The norm of the rocks as worked out above, omitting minor 
ingredients, corresponds fairly well with the mode in the norite of the 
basic side, except that monoclinic augite replaces some of the hyper- 
sthene. In the average acid phase of the eruptive pyroxenes hardly 
occur, being replaced by hornblende and biotite. 
CAUSES OF THE DIFFERENTIATION 
In my report on the Sudbury nickel region it was suggested that 
the differentiation of the ore from the rock, and of the norite from 
the micropegmatite, is mainly due to gravitation. The sulphides 
are of course far heavier than norite, and the norite is heavier than 
the micropegmatite; though in the latter case the difference in specific 
gravity is not so striking. When one observes how the ore has con- 
stantly accumulated in the hollows of the floor on which the norite 
rested, penetrating all the fissures of the rocks beneath, the conclusion 
seems irresistible that gravity played the main part in its segregation. 
