Presidential Addvess—The Geology of Devon. 561 
The above considerations tend to show that the relations of the 
granite to the surrounding rocks are somewhat peculiar, and that it 
is not exactly easy to frame a theory to satisfy all the conditions. 
The composition, which is that of a normal potash granite, and the 
contact phenomena, are clearly against the notion of any large 
absorption of the containing rocks, such as are now accessible to 
observation. ‘There has been much nonsense talked about granites 
being the result of the extreme metamorphism of the beds in 
which they occur. Mr. J. A. Phillips in his paper “On the Rocks 
of Cornwall in relation to Metalliferous Deposits”’ showed very 
clearly that, although the different kind of killas vary materially in 
composition, under no circumstances could the mere re-arrangement 
of the constituents result in the production of granite. 
_ He gives a table with the chemical compositions of ten varieties 
of Cornish killas, showing a range in silica from 38 to 68 per cent. 
and of alumina from 10 to 24 per cent. The alkali is mainly soda, 
and of this there is a considerable amount in some specimens, pointing 
to the conclusion that killas has been largely derived from the dis- 
semination of very fine volcanic matter of a basic composition. 
This coincides with the prevalence of contemporaneous volcanic 
phenomena. It is worth noting that the roofing-slate of Delabole 
affords an exception to this rule, in containing more potash than 
soda. 
As there is no reason to suppose that the early chemical history of 
the Dartmoor granite differs materially from that of the Cornish 
granites, their sources must have been deep-seated, and they must 
have originated under the ordinary conditions which produce the 
granitic magma, whatever those may be. The main questions re- 
maining to be considered are the period and circumstances under 
which the Dartmoor granite assumed its present position. 
There is no evidence at present, as far as I know, which would 
enable us to fix the period any nearer than the somewhat vague date 
“the close of the Carboniferous.” Dr. Barrois says that many of 
the Brittany granites are of Carboniferous age. But in the case of 
Dartmoor it is probable that the great foldings of the Hercynian 
mountain-system had been mainly effected, and the synclinal of 
Devonshire formed, before the granite was insinuated. In position 
the mass of Dartmoor is outside the axis of the Cornish granite ; 
if their alignment was followed the centre of Dartmoor would be 
about Hatherleigh. Hence the stratigraphical position differs some- 
what from that of the Cornish granites, although possibly their age 
may be quite the same. It is enough to know that an immense 
physical revolution was effected all over the British Isles between 
the close of the Carboniferous and the beginning of the Permian, 
and the intrusion of the Devon-Cornwall granites must have taken 
place either then or in early “red rock” times. 
Next, as to the circumstances under which the Dartmoor granite 
found its way into its present position. Last year Mr. Ussher treated 
1 Quart. Journ. Geol. Soc. vol. xxxi. p. 319. 
DECADE III.—VOL. VI.—NO. XII. 36 
