File:The Quarterly journal of the Geological Society of London (13205688485).jpg

SHARPE ON SLATY CLEAVAGE.
101
disturbing agent, and the connection between it and the effect it
has produced are obvious.
A more difficult case is that of an elevated area which has not
been broken through in the middle by any great igneous eruption ;
where we can only infer what was the nature of the disturbing cause
from observing the effects it has produced. This is the case of
the Wealden elevation which Mr. Hopkins has illustrated. This is
also the case of the elevations of Carnarvonshire and Merionethshire,
and of Devonshire and Cornwall ; for though in all these counties
abundant eruptions of igneous matter have taken place, some probably
connected with the elevation we have to consider, these are secondary
pheenomena resulting from a wider-spreading cause ; and in Devon-
shire and Cornwall the great granitic eruptions do not form part of
the case under consideration, for they were evidently hardened and
solidified on the surface before the elevation took place which was
connected with the cleavage and gave it that symmetrical arrange-
ment we now find.
The hypothetical case put by Mr. Hopkins, of a fluid mass below
the surface forced upwards along a rent bounded laterally by two
parallel walls, and covered over by the beds forming the crust of the
earth, is in itself probable, and will be found sufficient to explain
most of the phaenomena observed. The effects produced on the
mass above will depend on the width of the rent relatively to the
fluidity of the moving mass, supposing always, for simplicity of ar-
gument, that the fluid matter finds no means of escape between the
beds, but continues always to press upwards. If the rent is narrow
compared to the matter forced into it, the surface must give way in
a long crack through which the fluid will escape, as in the Malvern
Hills. If the rent is broader the fluid must force up the beds above
it in a curve, for the surfaces of the fluid mass must rise more in the
middle than at the sides.
Fig. 22.
Figure 22 represents a section of the supposed case; the lines
A B are the vertical walls of the fissure bounding a fluid mass which
can only escape upwards by displacing the mass above, of which the
part first opposed to it is shown in its original position by the line
C. The beds above will be successively raised into each of the
curves a c a: since the pressure is greatest in the middle of a fluid