Presidential Address—The Geology of Devon. 563 
But Mr. Worth himself has supplied evidence which goes far to 
explain the presence of remnants of felsitic, and even of volcanic 
rocks, in accumulations more recent than the “red rock” breccias. 
Such remnants are much more likely to have been derived from the 
elvans, which form so characteristic a feature in the country between 
the Dartmoor and Brown Willy granites, and some of which probably 
reached the surface in a more glassy condition than the portions now 
accessible to operations. Besides, even the existing dykes are 
represented in some cases as developing a semi-vitreous ground-mass 
with porphyritically imbedded crystals. 
Theoretically it is extremely probable that the granite bosses of 
Devon and Cornwall may have passed upwards into volcanic rocks, 
and that consequently they represent a line of eruptive vents 
which were possibly active in Permian times, or those immediately 
preceding. But the petrological evidence alone is not conclusive. 
If we suppose that the “red rock” breecias are of Triassic and not 
of Permian age, all, or nearly all, traces of the volcanoes might have 
been removed before the breccias were accumulated. Clearly the 
granite, with its characteristic crystals of orthoclase, had been laid 
bare when the beds containing Murchisonite were deposited. 
As regards the composition of the Dartmoor granite, the accessory 
minerals such as schorl, and the proneness of portions to kaoliniza- 
tion, are especially noteworthy. This latter feature has a tendency 
to produce unequal weathering, and it is not at all improbable that 
the Tors are in a great measure due to the unequal weathering 
brought about by this cause. They represent portions which, in the 
hour of trial, were harder and perhaps chemically more stable, and 
consequently less liable to disintegration. The forms of the Tors, 
as was pointed out by Prof. Rupert Jones and more recently by 
Mr. Ussher, have been largely determined by the arrangement of 
divisional planes, the mass being intersected by what the latter calls 
impersistent cracks, running more or less horizontally and crossed 
vertically or obliquely by joints. Variation in the direction of these 
joints is accountable for much of the variety in the Tors themselves. 
(4.) The Metalliferous Deposits—The abundance of schorl, espe- 
cially on the edges of the granite, and the kaolinization of the 
felspars, are indirectly connected with the last subject which it is 
proposed to bring to your notice; viz. the origin of the metalliferous 
deposits for which this region is so famous. 
It is, I believe, admitted that the great east-and-west fissures 
through which the elvanite has been injected were formed after the 
consolidation of the main mass of the granite, though their chemical 
composition points to their having been derived from the same 
magma as the granite. The next step in this curious underground 
history appears to have been the formation of a series of empty 
fissures, most of them having a more or less east-and-west orienta- 
tion. And now commenced a fresh set of phenomena which, in an 
extremely modified sense, may be said to be still in operation. 
The fissuring of this region was probably due to reaction after 
the strain consequent on the system of folding, to which allusion 
