TRANSACTIONS OF THE SECTIONS. 101 
are not separated from the Upper Ludlow by the usual Aymestry 
limestone. 
1. Blue unfossiliferous shale, rising conformably from under the 
Upper Ludlow, and passing into parallel thin beds of hard siliceous 
schist. These form the bed of the Dee for several miles above Llan- 
gollen, and their dip being in the direction of the stream, and their 
projecting edges being opposed to it, they give to its dark waters those 
alternate reaches of turbulence and repose which form the most inter- 
esting features of that celebrated river. 
2. The middle group consists of a great thickness of uniform parallel 
beds of light blue shale, some of which, on weathering, assume a 
whitish colour, and give the section a streaked or banded character. 
These form the bold promontory of Rhysgog, and are largely developed 
on the steep west face of the Wriddiog, by the side of the great Holy- 
head road. Their lower portion is interstratified with bands of hard 
sonorous greywacke, and passes into the 
3rd or lower group, consisting of the slates and flags which are 
quarried at the base of Cefn-uchaf on the south of the vale of Llan- 
gollen, and in the chain of hills on the north, as at Oirnant, &c., and 
are of great local and commercial value. At the north end of this chain 
the lowest beds repose upon the Lower Silurian rocks of Cyrn y Brain. 
The total thickness of the three groups is estimated at about 3,100 
feet. These rocks are compared with various recognised members 
of the same geological age, and of an intermediate character, from 
other localities, to show their connexion with the soft mudstones already 
alluded to; various other coincidences of dip, tendency to concretions, 
structure of joints and cleavage are cited. 
Mr. Bowman next described an elevated plateau of igneous rocks, 
occupying an area of about twenty square miles on the east flank of 
the Berwyn mountains between Llanarmon Dyffryn Ceiriog and Llan- 
saintfraid Glyn Ceiriog, not noticed in the latest geological maps. It 
is divided by a deep picturesque gorge, through which the river Ceiriog 
and the road between the two villages just named, pass. The igneous 
matter varies from a pure white compact felspar to a gray or greenish 
trap, which in places is stratified, and resembles grauwacke. It has 
thrown off the sedimentary rocks on all sides, and burst through them 
in various places, forming insulated hills of trap. Near the centre of 
the plateau, at the top of the hill of Pen y Craig, is a column of com- 
pact white felspar 20 to 25 yards deep and 16 to 18 yards wide, hem- 
med in on each side by a wall of Lower Silurian rocks, which it has 
rifted asunder, and overspread laterally to a considerable distance, its 
hardened perpendicular masses crowning the precipice like a ruined 
castle. Blocks of this felspar strew the surface to a considerable dis- 
tance, mixed with others of stratified and amorphous trap; and others 
project from the rifted sides of the gorges. Some of the stratified 
traps so repeatedly alternate with the schists, that it is difficult to avoid 
the conclusion of their having been formed simultaneously ; while 
others seem to have been forcibly injected between the hardened beds, 
and to have taken the shape of the intermediate spaces. The rifted 
