Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/263

Rh the direction of the second pressure may coincide with that of the first; and though perhaps the result may be more intense cleavage in the previously cleaved rocks, no crossing of cleavage-planes could be produced; and, moreover, in cases where the second cleavage is very intense, or the rock very susceptible of cleavage, this would entirely obliterate all previously existing divisional planes, as notably in the volcanic slates of the Lake-district, where we find joints and faults quite healed, and the rock splitting along the cleavage-planes only.

The beds below the Grits are, as a rule, bluish rab, i. e. a shivery mudstone breaking along bedding, joints, and imperfect or double cleavage into irregular, often somewhat prismatic fragments. Frequently, however, these beds have, especially where much weathered, a pale grey colour, much resembling that of the beds above the Grits. This may perhaps be because they are derived directly from the felspathic ash of some volcanic district, while the Silurian beds above the Grits have been formed later on from the waste of such beds.

These pale beds above the Grits pass up into the "Pale Slates" of the Survey, which in turn pass up into the striped flaggy Slates of Penyglog, on the top of which come Grits to be referred to the true Denbigh Flag- and Grit-series, and which I traced for about two miles to the N. side of Moel Ferna. In the flaggy Slates I found Graptolithus priodon, Cyrtograpsus Murchisoni, and Retiolites, sp., with Orthoceras primævum and O. subundulatum. I again found some Graptolites in a small watercourse N.E. of Moel Ferna, on what seemed to be the same horizon. The dip is here about 10°, N.E. to N.N.E., and the cleavage 30° N. I would here acknowledge much useful information which I got from Mr. Phillips, of the Penyglog Quarries.

I then tried to apply this key to other districts.

First, then, there is a patch of Bala coming in south of the fault near Bryn Gorlan, at the south end of the Yale of Clwyd, as shown in the section (fig. 2).

Here I found at the north end the Denbigh Flags. These may be examined along the road to Llanfairdyffrynclwyd, where Graptolites (chiefly G. priodon) occur. South of the Bryn-Gorlan fault we