1815.) Geological Observations on North Wales, 365 
ranges of columns, It appeared to me to lie in strata which dip 
towards the S. 
North of the Cader Idris chain a lower range of hills runs nearly 
parallel with it. The river Mowddach flows through the intervening 
valley. These hills consist, at least along the banks of the Mowd- 
dach, of a rock composed of felspar, and containing in some places 
large crystals of green hornblende, which alternates with clay-slate. 
At Rhaidyr di a section of the rock has been formed by a torrent, 
where many of these alternations present themselves. These beds 
dip to the S., towards the direction of the Cader Idris chain. The 
same rock alternating with slate prevails very extensively between 
this place and the Snowdon range. The hornblende is often wanting. 
' I ascended Snowdon from Capel Curig, but on that side could 
distinguish nothing with respect to the disposition of the rocks. 
The porphyritic rock, resembling that of Cader Idris, extends quite 
to the base on the south-eastern declivity. But on going up on the 
northern side from Llanberris, I found a tolerably distinct appear- 
ance of strata, with an inclination towards the S. From the foot 
f the mountain to about half way up, I traced frequent alterna- 
tions of clay-slate and porphyry. About two-thirds of the way up 
a precipice of considerable extent fronts the N., where the rock, 
though split by fissures in all directions, appears to be stratified, as 
before mentioned. The top of the mountain consists of porphyry. 
In several other parts of the Snowdon chain, which I traversed in 
various directions, the appearance of stratification was more distinct 
than in the mountain which is properly called Snowdon. This I 
observed particularly between Capel Curig and Llyn Idwal. There 
are many quarries of roof slate in this range ; and one of consider- 
able extent, in a hill which is separated from Snowdon by the, 
narrow valley of Lianberris. 
_ This valley presents a beautiful section of the chain. Magni- 
ficent ranges of porphyritic rocks strike the eye on both sides, and 
yy their columnar forms have given rise to the mistaken idea that 
these hills, as well as Cader Idris, are covered by basaltic rocks. I 
aw no trace of basalt on any part of them, neither could 1 discover 
y granite or mica-slate. 
_A question here presents itself whether the porphyritic rock 
hich forms the top of Snowdon belongs to the overlying forma- 
n, or is conformable with the strata of clay-slate which constitute 
e base of the hill. ‘This inquiry is, as it will presently appear, of 
onsiderable importance in assisting to determine the era to which a 
at part, if not the whole, of the slate formation in North Wales 
nust be referred. From the huge tabular and columnar masses into 
which the porphyry is divided, and from its occupying more re- 
markably the tops of the mountains, 1 was at first led to refer it to: 
the overlying formation ; but the frequ:nt alternations of the same 
porphyry with the clay-slate at the base of Snowdon, and other 
lls in the range, seem to countenance the opinion that it holds a 
position conformable with the clay-slate. Any person’ who would 
al 
