F. Divey—The Magnesian Growp of Igneous Rocks. 491 
A release of pressure over any given portion of the infra-pltutonic shell 
would allow the liquefaction of that portion cf the shell under the high 
temperatures prevalent ; such liquid rock, on being iftruded into the 
higher portions of the earth’s crust, would then solidify under less 
pressure as a plutonic rock. But in these circumstances, doubtless 
comparatively rare, in which the isogeotherms are lowered more rapidly 
than the pressure, it can easily be imagined that the rock would be avle 
to crystallize in its infra-plutonic form, and then, by subsequent earth 
movements and denudation, be exposed at the surface.t 
Stillwell, however, in opposition to this view, states? that “ the 
‘infra-plutonic zone’ is indistingnishable from a metamorphic 
zone defined by the particular set of conditions ”’, and he endeavours 
to show, moreover, that the garnets, the granularity,® and, in fact, 
all the peculiar features of these rocks, are due to recrystallization 
in the solid state. In extension of this idea, he even argues * that 
the following hypotheses, ‘‘ founded on the transition between 
two rock types in metamorphic areas ”’, might be explained “ on the 
hypothesis of metamorphic diffusion”, which he also regards as 
operating in the solid state :— 
1. Intermingling of basic magmas with acid magma. 
2. Differentiation of an intermediate magma into a relatively 
basic portion and a relatively acid portion. 
3. Local melting or refusion in situ. 
4. Gradual assimilation of pre-existing basic sediments by 
invading granite or gneiss. 
It would indeed be strange if these ancient rocks had not 
suffered some metamorphism at least, but the evidence brought 
forward to show that metamorphic diffusion in the solid state 1s 
capable of producing such remarkable changes in the character of 
the rocks is far from being convincing. 
Dr. J. W. Evans considers the presence of abundant hypersthene 
in charnockitic rocks as due to the influence of excess alumina, 
which he regards as having been derived by the absorption of 
aluminous sediments.> So far there does not appear to be very much 
direct evidence in support of this theory in the various areas con- 
sidered in this paper; in one of these areas, at least, namely the 
1 See also Helland on garnets in ancient rocks of Southern India, above, 
p. 485. Moreover, E. Weinschenk has brought forward evidence to show that 
the garnets, as well as the granularity, of the central granitic gneisses of the 
Alps may be regarded as having been produced during the crystallization of 
a viscous magma under the operation of a powerful directed pressure (“ piezo- 
crystallization”), and not by later metamorphism. Cong. Géol. Inter., C.R., 
vili, 1900, p. 326. 
2 Op. cit. supra, p. 196. 
3 Granularity cannot necessarily be regarded as evidence of metamorphism ; 
many igneous rocks are known to possess well-developed granular texture as a 
result of disturbance of some kind during crystallization. See Weinschenk re 
“‘ niezocrystallization”’, op. cit. supra, and also F. Dixey, “The Norite of Sierra 
Leone”’?: Proc. Geol. Soc., 1921, p. 103. 
= Ojos Ollie, yo, UPAlle ° 
5 Abs. Proc. Geol. Soc., 1921, p. 46. 
