$64 Geological Observations on North Wales. [Nov. 
towards the north than the south side. The declivity towards the 
south is often gentle, while the northern face is precipitous, © This 
remark, however, is not universal. j 
The inclination of the strata is various. In many places they are 
vertical ; but the dip most prevalent threugh the country appears to 
be towards the S. and S.W. This circumstance, compared with 
the position of the escarpments of the hills, indicates that in ad- 
vancing from S. to N. you pass continually from newer to older for- 
mations. 
On passing from the tract of the old red sand-stone in Hereford- 
shire, the first rocks which present themselves are decidedly of the 
transition formation. Near Ludlow, in Shropshire, -beds of clay- 
slate first occur alternating with thin beds of blue lime-stone, con- 
taining numerous impressions of small shells. From this place to 
Montgomeryshire clay-slate abounds. This I observed on my re- 
turn. I entered North Wales through Brecknockshire; and the’ 
first transition rock which attracted my notice, and which appears 
immediately to succeed the old red sand-stone, is greywacke-slate, 
which about ten miles S. of Buallt forms magnificent cliffs on both 
sides of the river Wye. It splits into slates half an ineh thick, and 
contains a large portion of mica. The beds dip at a considerable 
angle towards the S. E., and the hills form bold escarpments-front- 
ing the N.W. 
Near Buallt the hills consist of a crumbling clay-slate resembling 
slate-clay. In the bed of the Wye, about three miles N. of Buallt, 
T found greywacke in mass alternating with a hard clay-slate. ‘The 
former consisted of small fragments of quartz and clay-slate im- 
bedded in a hasis of clay-slate. 
From Rhaiadyr to Hafod, and to Plinlimmon, clay-slate pre- 
vails.. At the Devil’s Bridge it is quarried, and furnishes a fine blue 
roof slate, very hard, and of considerable lustre. ‘The Piinlimmon 
chain consists of the same rock. Large veins of snow-white quartz 
traverse it in all directions, loose blocks of which are scattered on 
the tops of the mountains, ’ 
The chain of Cader Idris bears a great resemblance in the com- 
position of its rocks to that of Snowdon. I ascended the former 
from Tal y Llyn by the side of Llyn y Cae. About half way up the 
mountain I found beds of clay-slate and of flinty-slate dipping at’ 
an angle of 75° to the S._ The top of the hill consists of a beautiful, 
clay-stone porphyry, containing crystals of felspar and some quartz 
imbedded in a basis of hard clay-stone, which passes into horn-stone. 
In some places the form of the felspar crystal is wanting, and the 
porphyry is amygdaloidal. It forms large tabular masses and 
columnar rocks, which affect a pentagonal form, I should have 
considered the upper part of this hill as an example of the overlying 
formation, if I had not found some facts in Snowdon which dispose 
me toa different opinion. The northern side of Cader Idris pre- 
sents a bold precipice, in which the porphyritic ~ock forms beautiful 
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