386 SECTIONAL TRANSACTIONS.—C. 
is competent to produce granite in bulk commensurate with the West of England 
complexes and others enormously larger elsewhere. Moreover, basaltic parentage 
does not lead us to expect that a granitic end-product is likely to be specially enriched 
in tin, tungsten, and tourmaline. But the Hercynian (dominantly alkalic) granites 
of the West of England are so enriched ; whereas the Caledonian granites (calc. alkali) 
are essentially barren. ‘To explain these contrasts we are at liberty to assume either 
(a) contrasts in the composition of the initial basalt-magma, or (b) differences in the 
tectonic or other controls affecting the trend of differentiation, or (c) some combination 
of both factors. Any one of these explanations may be quite valid, but the scanty 
ad hoc data available warrant neither assertion nor denial: the problem is essentially 
untouched. 
But differentiation alone, working on presumably fixed quantities of initial basic 
magma, does not cover the whole case so far as the petrologist is concerned. The 
problem engages both assimilation and palingenesis, the effects of which may serve 
to explain some of the many departures from the general rules associating magmatic 
ore-deposits with igneous rock-types. 
Both assimilation and palingenesis are accorded due recognition by Petrology as 
a Science; but not all petrologists are willing to acknowledge the quantitative 
importance of either process. On the other hand, Prof. J. J. Sederholm attributes 
some of the Archzan granitic massifs of Fennoscandia to palingenesis, and Von Bubnoff 
believes that the Hercynian granites and diorites of the Schwartzwald and Odenwald 
originated by the regional fusion of the bases of ancient gneisses. In a personal 
communication to the writer, Prof. A. Holmes has suggested that the Hercynian 
granites, from the West of England to Spain, originated by palingenesis in the roots 
of deep-sunk orogenic folds. In the mass of data concerning the West of England 
granites in particular there is nothing inconsistent with this view; on the contrary, 
there is much to support it. . 
At any stage in the process of differentiation, substantial additions can be made 
to the material undergoing differentiation: acid magma, whether evolved or 
regenerated, may be basified, and basic magma, whether primary or derivative, may 
be acidified. The added material may be igneous, sedimentary, or metamorphic, 
and in each case the introduction of ore-metals and fluxes into the magma is a possi- 
bility. The bearing of this fact on the problem of magmatic ores is obvious. 
The basification of acid magma is convincingly demonstrated by occurrences 
recently described by Drs. A. K. Wells and 8. W. Wooldridge (1931: Ronez, Jersey), 
Dr. H. H. Thomas and Mr. W. Campbell Smith (1931: Trégastel-Ploumanac’h, Cotes 
du Nord), Mr. S. R. Nockolds (1931: Dhoon, Isle of Man), and Dr. D. R. Grantham 
(1928: Shap, Westmorland). Earlier proofs include the classic cases described by 
Prof. W. J. Sollas (1894: Barnavave Hill, Carlingford), Dr. A. Harker (1895: Carrock 
Fell, Cumberland), Prof. A. Lacroix (1898-1900: Quérigut, &c., Pyrenees), Messrs. 
F. D. Adams and A. E. Barlow (1910: Haliburton-Bancroft district, Ontario), and 
Prof. 8. J. Shand (1921: Sekukuniland, Transvaal). The acidification of basic magma 
has been described by Dr. H. H. Thomas (1922, Mull), Prof. H. H. Read (1923: 
Aberdeenshire), Messrs. A. L. Hall and A. L. du Toit (1923: Bushveld), &c. Both 
these lists could be considerably extended. Hybridisation processes receive extended 
treatment in Prof. Daly’s well-known work (1914), and representative cases are 
critically reviewed by Prof. Shand (‘ Eruptive Rocks,’ 1927). 
The hybrid character of the main biotite-granites of Dartmoor is demonstrable 
from field evidence, petrological fact, and analytical data alike. The composition of 
each facies satisfies the generalised equation :— 
Biotite-granite—m per cent. peraluminous acid magma-+n per cent. assimilated basic 
igneous rock approximating to diabase. 
In this equation the value of m ranges from 2:5 to not more than 16°3; the 
“peraluminous’ character of the ‘acid magma’ provides, in part, for the felsic 
constituents derived from assimilated shales. Variation, which engages the silica- 
range 55 per cent.—75 per cent., recapitulates the normal phenomena commonly 
attributed to differentiation alone, the closing phases being characterised by 
tourmalinisation, greisenisation, kaolinisation, and the formation of lodes. But this 
variation is a bi-generic feature, differentiation and assimilation haying proceeded 
simultaneously until low temperature and high viscosity put an end to both. 
