486 F. Diney—The Magnesian Group of Igireous Rocks. 
India, although characterized in all members by hypersthene, 1s 
especially rich in this and other magnesian minerals in the less 
acid members. Recent work of the Geological Survey of Sierra 
Leone! has shown that these two series of rocks, in addition to 
huge masses of talc-schist, amphibolite, etc., are well represented 
in Sierra Leone also. 
Thus it is seen thet Southern India islargely composed of enormous 
masses of rocks belonging to the Charnockite Series, of associated 
peridotites and their altered equivalents, and of later noritic and 
pyroxenic dykes, and also that the older igneous rocks of at least 
the central and western parts of West Africa consist of a similar 
assemblage ; these two areas may be considered as exhibiting 
typical developments of the group of igneous rocks now under 
consideration. A number of analogous areas will be cited in a later 
paragraph. 
II. DeEscripTIOoN oF THE MAGNESIAN GROUP. 
Since the principal characteristic of these rocks is their richness 
in magnesium, it is proposed to distinguish them as a Magnesian 
Group of igneous rocks.2 The term “ group ”’ is used in preference to 
“ series’ because, as will be shown below, the rocks in question 
include some or all of the members of certain series already 
recognized, and also because it is at present uncertain how far they 
form a continuous series in themselves. The rocks of this group 
give rise in various parts of the world to a limited number of 
Magnesian Provinces, of which West Africa may be taken as a good 
example, as indicated in the preceding paragraph. The term 
“Province”? is here used essentially in a geographical sense, 
although, as will be seen below, the principal members of the group 
are of Archaean or very early age; moreover, although the rocks 
of any one of the provinces do not necessarily belong to one igneous 
epoch, they are, nevertheless, related in important mineralogical 
and chemical characters. 
It is, of course, recognized that highly magnesian rocks have 
occurred in all ages as differentiates of large intrusions of less basic 
composition ; in the present paper, however, consideration is given 
essentially to the large magnesian intrusions, such as those ot 
India referred to above, that apparently possess a separate existence. 
In all probability differentiation of some kind operated even in the 
production of these large intrusions, but this differentiation took 
place on a very large scale indeed, giving rise to the great crustal 
“ shells ” referred to below. 
1. Dixey, Reps. Geol. Surv., Sierra Leone, 1919-20. 
2 Note.—An analogous arrangement has been suggested by Washington 
with regard to a small group of rocks, characterized by high content of titanium, 
which he proposes to distinguish as a “‘ Titanium Province”. See H. 8S. 
Washington, ‘‘ The Titaniferous Basalts of the Western Mediterranean ”’ : 
Q.J.G.S., vol. Ixili, 1907, p. 69. . 
