1822.] Geology of Snowdon, and the surrounding Country. 409 
ing through the almost unbroken verdant surface of this side of 
the mountain. These slates appear to be uniformly of the usual 
slate blue colour, and not to differ perceptibly from the common 
variety of the killas of the Cornish miner ; but just as we reached 
the ridge above Cwm Clogwin, the slates began to assume a 
different aspect and character, to become paler and more granu- 
lar. We afterwards crossed a bare and somewhat elevated ridge 
of rock almost perfectly resembling some of the varieties of the 
base rock of Moel Shabod, being granular, and somewhat slaty, 
but sufficiently soft on its broken surfaces, which are very irre- 
gular, to yield to the pressure of the nail, and producing a very 
copious effervescence with muriatic acid. We consider this 
rock to be an intimate mechanical mixture of steatite and car- 
bonate of lime, since the latter does not appear in separate 
masses or layers. Slates again prevailed, and then arock some- 
what slaty greatly resembling some fine-grained varieties of 
greywacke in its external character, but soft enough to yield 
readily in every part to the knife, and of which we consider the 
base to be steatitic: it also effervesces. That which succeeds 
this rock is a slate, yielding easily to the knife, and even, 
though with some difficulty, to the nail in its moist state. It 
partakes greatly of the nature of the slate just mentioned as 
occurring at the foot of Moel Eilio, but more nearly approaches 
common slate in colour and general aspect. 
It must here be observed, that the cleavage plane of these 
slates and slaty rocks is nearly vertical, and in the usual direc- 
tion of NE and SW; and that our walk was precisely across the 
cleavage at every step. Here is a small slate quarry. The 
guide told us that we had advanced just half way up the moun- 
tain. 
A vast heap of ruin succeeded, consisting of much harder 
rocks than any we had before seen on the mountain, but man 
of them agreeing in character with the prevailing rocks of the 
base of Moel Shabod, and among them some which occasionally 
appeared altogether homogeneous, sometimes slightly porphy- 
ritic, the base being of a greenish colour, very hard, scarcely 
yielding to the knife, and containing minute crystals of felspar. 
This rock perfectly resembles the hard nodules already described 
as occurring abundantly in the steatitic rock near the foot of 
Moel Shabod opposite to the back of Capel Curig Inn. These 
often assumed the form of short irregular columns. Nearly 
vertical slates (as regards their cleavage) succeeded, and then a 
nearly vertical and somewhat slaty rock, which is hard enough 
to scratch glass, is translucent on the edges, and by transmitted 
light appears to contain minute specks of a green substance, 
probably chlorite, but superficially it appears homogeneous. 
Still ascending the eastern ridge of Cwm Clogwin, we found 
slaty rocks again prevailing (the cleavage being indistinct and 
occasionally somewhat curved), but very soft, of a dark-green 
