File:The Quarterly journal of the Geological Society of London (13205993984).jpg

92 PROCEEDINGS OF T«& GEOLOGICAL SOCIETY. .DeC. 2,
suggestion, as the lines appear to correspond to portions of curves
having a common axis. But, as each of the planes runs in the same
position for a great distance, if the curves were completed they would
represent a series of semicylinders turned over a common axis*.
The arrangement of the planes of cleavage on so symmetrical a
plan is in itself deserving of attention ; but when their direction is
compared with the position of the beds over the same area, we find
coincidences too remarkable to be due to accident, and which lead
to considerations of great importance connected with the elevation
and disturbance of portions of the crust of the globe. The remarks
which follow will be made intelligible by reference to my map of part
of North Walesf, and to the sections figs. 17 and 18, in which the
"^ ' Figs. 17 and 18.
1. Snow don. 2. Rhaiadr Cwm. 3. Bala.
minor irregularities are omitted and only the main features preserved*
to prevent the attention from being distracted by minutiae all subor-
dinate to the larger phsenomena. Fig. 17 represents the position
of the beds along a section of the country from the W. side of Snow-
don to the slate bed E. of Bala, fig. 18 the dip of the cleavage planes
along the same line of section.
At Rhaiadr Cwm, which is on the axis of the elevation of all the
planes of cleavage of the area, there is an anticlinal axis of the stra-
tification along which the oldest rocks of this part of North Wales
are brought to the surface in a low flat arch J, which forms but a slight
feature in the country ; this now proves of great significance, as it
must be considered the true axis of elevation of the whole area, both
of the cleavage planes and the stratification. We cannot trace this
axis for more than a few miles either to the N.E. or the S.W., as in
both directions it is entangled in great masses of erupted igneous
rocks. In Professor Sedgwick's map of North Wales §, the line laid
down as the "great Merioneth anticlinal" is parallel to the axis at
Rhaiadr Cwm, but is drawn five miles to the S.E. of it.
On both sides of this central axis, there is a relation between the
inclination of the cleavage planes and the elevation of the strata :
where the dip of the cleavage is at the lowest angle, the elevation of
the beds is most moderate and they are but slightly disturbed ; where
the cleavage dips at a higher angle, the beds are more elevated and
disturbed and their dip occasionally reversed ; and near the boundaries
'*• * Although in theory we ought to regard the cleavage as arranged in curves, I
have continued to use the term cleavage jy/anes to which we are accustomed, from
dislike to introducing a new term, and because to our senses they appear planes, and
not curves.
t Quarterly Journal of Geological Society, vol. ii. pi. 12. xltefs'i airiT *