THE DARTMOOR VOLCANO. 
157 
the size of Dartmoor, three to four hundred square miles in area, 
and from existing data at least half a mile thick, forming a 
subterranean lake, without finding its way to the surface some- 
where, and putting on volcanic characters. 
D. There remains therefore only the volcanic hypothesis. 
The point on which I wish to lay special stress is this — that 
there must have been a period in the eruption of the Dartmoor 
granite when the superincumbent mass ceased only to be lifted, 
and began to be thrust. Coming up from below — as the granite 
must have done from its present contour — partially in a wedge- 
shaped or conical form, the first erupted material leading the way 
in the line of greatest weakness, the Devonian and Carboniferous 
rocks must eventually have been driven apart as well as heaved. 
This thrust would become stronger as the erupted body increased 
in mass, and it would continue to grow in force until the tension 
near the surface was relieved. The lower rocks would be the 
first to feel the separating action. Hence upheaval would be quite 
consistent with Devonian and Carboniferous strata still retaining 
their relative positions on the flanks. 
There is absolute proof that something of this sort took place, 
in the fact that the Carboniferous and Devonian rocks about 
Dartmoor, the latter especially, now occupy, by tilting and re- 
petition, a much narrower lateral area than originally. They were 
once horizontal, or approximately so ; they are now mainly inclined 
at high angles and plicated. 
Some of these rocks, however, were simply lifted ; where they 
were caught in a hollow of the irregularly uprising granite. 
There are talcose slates, for example, approximately horizontal 
near Slade, in the lap between the main granitic mass and the 
Crownhill island, saved from tilting by the protecting elevations 
on either side. 
It will suffice if I confine my illustration of the effects of 
thrust to the Devonian rocks between Dartmoor and Plymouth 
Sound. They have not only been tilted, but thrown into a series 
of folds, partially identified by the repetition of certain beds ; so 
that the nine miles or so of Devonian rocks between Shaugh and 
Wembury must contain, at the very lowest estimate, strata that 
originally extended over twelve or fourteen. These thrust effects 
are thus well marked in this locality, because there was a great 
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