F'. Dixey—The Magnesian Group of Igneous Rocks. 487 
The distinguishing features of the Magnesian Group may be 
summarized as follows :— 
Texturally, these rocks are often granular, and they are generally 
well-banded. 
Chemically, the rocks are rich in magnesium and in iron, 
especially in the less siliceous varieties; the abundance of these 
two constituents, combined with fairly low alumina and lime, leads 
to the persistence of hypersthene. There is wide variation in the 
content of silica. 
Mineralogically, the rocks are almost invariably characterized 
by the presence of hypersthene, and a monoclinic magnesia- 
pyroxene sometimes accompanies and may even replace the 
hypersthene ; moreover, common augite is generally an important 
constituent. Olivine is often abundant in the more basic members 
of the group. Garnet is a common and sometimes abundant con- 
stituent, particularly amongst the older rocks. 
Concerning the presence of garnets in the Charnockite Series 
of India, an important member of the Magnesian Group, Holland 
has stated 1 :— 
The prevalence of garnets in some members of the Charnockite Series 
which show no signs of dynamo-metamorphism, suggests that although 
pyroxene is the stable form of the stme compound at low temperatures, 
there is an intermediate but high temperature, short of fusion, at which 
either hornblende or pyroxene breaks up into a more basic garnet and a 
more siliceous bye-product, quartz or felspar. It is to such a cause that 
the prevalence of garnets in these o!d pyroxenic rocks should be ascribed.* 
Stillwell,? however, regards the peculiar mineralogical features of 
the Charnockite Series as due to recrystallization in the kata zone 
of metamorphism (Grubenmann) ; he considers that not only the 
garnet, but even the hypersthene, was produced in these circum- 
stances. This does not accord with the obvious and almost 
unmodified igneous character of many of the rocks, and the present 
writer considers that the hypersthene at least is essentially a normal 
product of crystallization from the magmas, and that at least much 
of the garnet arose in this way. Garnet is now well-known to be 
a product of crystallization of many igneous rocks,? and Stillwell 
himself describes,®> from certain rocks in Adelie Land closely 
analogous to the Charnockite Series, the case of a garnet mass 
which occurs graphically intergrown with quartz in pegmatite 
cutting a granodiorite gneiss free from garnets. 
1 Op. cit., p. 196. See also Fermor below, p. 490. 
2 See also Fermor on the ancient garnetiferous rocks of India, below, p. 490. 
3 ‘Metamorphic Rocks of Adelie Land’’: Rep. Aust. Ant. Exped., vol. iii, 
1918, pt. i, p. 18. See also below, p. 489. . 
4 In this connection it may be noted that Dr. J. W. Evans has recently 
postulated the formation cf garnets, together with certain other minerals of 
small molecular volumes, in magmas crystallizing under conditions of high 
pressures and comparatively low temperatures. ‘‘ The Origin of the Alkali 
Igneous Rocks”? : Guon. Maa., Vol. LVIT, 1920, p. 559. See also note below, 
p- 491, on garnets and “ piezocrystallization ”’. 
> Op. cit., p. 197. 
