490 F. Diaey—The Magnesian Group of Igneous Rocks. 
of the following three shells which underlie, in downward succession, 
a discontinuous sedimentary sbell :— 
1. Granitic shell. 
2. Transitional shell. 
3. Basaltic shell. 
Finally, as has been suggested by several authors’ from con- 
sideration of the composition of meteorites, it is highly probable 
that the basaltic shell is underlain by a peridotic shell. If any such 
view be accepted, the known restriction of the majority of the 
magnesian rocks to the older parts of the earth’s crust and to very 
early times receives.some explanation ; in the first place, the deep- 
seated, highly magnesian magma of the lowest shell would be less 
likely to reach the higher parts of the crust than the overlying less 
magnesian rocks; secondly, in the course of time these erupticns 
would get less and less chance of reaching higher levels, both because 
the solid crust would probably be increasing in thickness and 
because, with continued density stratification, the magnesian 
magmas would become more and more inaccessible ; and thirdly, 
the rocks once situated at a great depth would now be exposed at the 
surface only in those areas, such as Southern India ? for example, in 
which uplift and progressive denudation have proceeded more or 
less uninterruptedly over a very long period of time. These ancient 
magnesian intrusives were frequently of very great size, and in this 
connection Holland has observed?: “‘ There is a strong probability 
that the magmas tapped in earlier geological periods -were larger 
than those which gave rise to eruptions after the earth’s crust had 
advanced in physical differentiation.” The great variation in 
acidity shown by some members of the Magnesian Group may be 
explained by intermixture of the magnesian magma with varying 
amounts of the overlymg magmas. That this intermixture would 
often be considerable may be assumed from the extraordinary 
development of igneous activity of all kinds in earlier periods of the 
earth’s history. 
Fermor,‘ in an attempt to explain the common occurrence of garnet 
in the ancient rocks of India, and noting that the formation of this 
mineral in a rock is accompanied by a reduction in volume, came to 
the conclusion that the garnetiferous rocks originate in a relatively 
basic “infra-plutonic zone” of great depth and unusually high 
PreueUre: 5 He states,® moreover :— 
“a Daly, op. cit. Also L. L. Fermor, Rec. Geol. Surv. India, vol. xtii, 1913, 
p. 41. 
2 T. H. Helland, ‘“‘ The Charnockite Series’”?: Mem. Geol. Surv. India, 
xxvii, 1900, p. 196. 
3 Op. cit. 
ries Preliminary Note on Garnet as a Geological Barometer and on an 
Infra-pJutonic Zone in the Harth’s Crust” : Rec. Geol. Surv. India, vol. xlit, 
1913, p. 41. 
5 Cf. also Dr. J. W. Evans; see footnote on p. 487 above. 
8 Id., p. 44. 
