F. Dixey—The Magnesian Group of Igneows Rocks. 493 
between rocks of the Magnesian Group and the basic members of 
the Alkalme and Calc-alkaline Series; it is obvious that strong 
resemblances exist amongst these rocks. The Charnockite- 
Anorthosite Series and associated rocks, forming the most impertant 
- members of the Group, are for the most part of pre-Cambrian age, 
whereas the other series referred to include types that are con- 
siderably younger. 
A careful comparison of the chemical and mineralogical features 
of the three series as they are represented in West Africa has been 
made by Lacroix! He has shown that the rocks of both the 
Charnockite-Anorthosite Series and the Calc-Alkaline Series are 
rich in magnesium and also in hypersthene, besides containing 
variable amounts of the magnesia-pyroxene. 
The Spilitic Suite, proposed by Dewey and Flett,? is distinguished 
from both the Alkaline and the Calc-Alkaline Suites maimly by 
abundance of soda-felspars and by association with special con- 
temporaneous geographical conditions; in the opinion of several 
later writers,® however, the Spilitic Suite is essentially a modification 
of the Calc-Alkaline Suite. Accordingly, the remarks already made 
as to the relation of the Allkalie and Calc-Alkaline Series to the 
Magnesian Group may be applied also to the Spilitic Suite. 
Again, Wolff * has suggested that certain basalts and basaltic 
rocks should be grouped together under the term “ Arctic Tribe ”’, 
and he has further proposed to include the Charnockite Series in this 
group. Washington,’ however, in a careful discussion of the 
Charnockite Series of India and a comparison of these rocks with 
analogous types of other regions, shows that the charnockitic rocks 
are clearly distinguishable from the ordinary basaltic or gabbroic 
rocks. 
In conclusion, it may be mentioned that the conception of the 
Magnesian Group of Igneous Rocks is based on evidence that 
increases with our growing knowledge of the older parts of the earth’s 
crust, and it is accordingly now put forward in the hope that it 
expresses a principle which may possibly be of some slight assistance 
in the general problem of the origin and relations of igneous rocks. 
1“ Les Syénites Nepheliniques de lArchipel de Los”: Nowv. Arch. du 
Mus. d@ Hist. Nat., Paris, 5me Série, Tome 3, 1911, p. 108. 
2 “Some British Piilow-Lavas and Associated Rocks”’: Gzou. Mac., 1911. 
3 See R. A. Daly, Igneous Rocks and their Origin, 1914, p. 340. 
4 F. v. Wolff, Vulkanismus, i, 1914, p. 153. 
° “ The Charnockite Series of Igneous Recks’?: Amer. Journ. Sci., 1916, 
Doze: 
