560 Correspondence—C, EH, Tilley. 
that hypersthene appears instead of biotite, as in the equation 
biotite + quartz os hypersthene -++ orthoclase. 
The presence of garnet (almandine) in the more acid types at least, 
is also an expression of a low active mass of water to which may 
be added the influence of high pressure (Q.J.G.S., 1921, p. 94). 
While the occurrence of hypersthene in acid charnockitic rocks may 
thus be ascribed to a comparative poorness in magmatic water, 
it is of interest to note that hypersthene is also found in acid rocks 
which otherwise cannot be included in this group. In such abnormal 
cases its presence can be ascribed to hybridism. This is the case 
with certain acid gneisses in the island of Rum, described by 
Dr. Harker. The conversion of olivine to enstatite (hypersthene) 
supplies an explanation in some cases where olivine is a constituent 
of the absorbed material. The monoclinic pyroxene of acid igneous 
rocks is characteristically a non-aluminous type, and it is not 
improbable that an aluminous type is unstable in the presence of 
quartz. Where acid rocks have been contaminated with basic 
material such as gabbro, the aluminous pyroxene of the latter might 
be expected to react with silica as in the equation :— 
{ neal \ 4 38i0, = xCaMgSi,0, + 2Mg0,A1,0,, 5Si0,) 
with the production of cordierite. But, as is well known, in contact 
metamorphism cordierite and diopside are mutually exclusive, and 
the products of this reaction under hydrostatic conditions would 
probably react with the production of anorthite, enstatite 
(hypersthene), and quartz, thus :— 
2CaMgSi,O, + Mg,Al,Si,0,, = 2 CaAl,Si,0, + 4MeSi0, + Si0,. 
Cordierite, being a common product of thermai metamorphism, 
it might be expected that hypersthene would similarly arise if 
diopside were present in the invading magma, by assimilation of 
aluminous sedimentary material. In this modified sense there may 
be some justification for Dr. J. W. Evans’ contention, mentioned by 
Mr. Dixey, when applied to cases where bybridism can be proved on 
other grounds. It is of interest to note in this connexion the example 
afforded by the norite intrusions of Huntly, Aberdeenshire, described 
by Mr. W. R. Watt (Q.J.G.S., 1914, p. 266). Monoclinic pyroxene 
is &@ normal constituent of the norite, but where the latter has been 
subject to contact metamorphism or has intruded sediments with 
. transference of material, cordierite appears, but diopside is con- 
spicuous by its absence. 
= C. EK. TIntey. 
