1822.] Geology of Snowdon, and the surrounding Country. 331 
the whole of the time we were in the Pass, and sometimes fell 
very heavily ; we did not, therefore, reach Llanberris. 
Having been informed by Mr, Dawson that he had discovered 
a trap dyke on the very summit of the Pass, and that we could 
not descend without treading upon it, we sought for it with 
much attention, and repeated our search upon a subsequent 
visit, but in vain. The summit appeared to us to consist chiefly 
of alluvial matter, among which we found, and especially in the 
horse road, several specimens of a rock much resembling decom- 
osing greenstone or trap, not very uncommon upon Moel Sha- 
Bod, and of a similar character to the decomposing rock on 
Cader Idris, which has given rise to the notion that volcanic 
debris are discoverable on the latter mountain: the cellular 
appearance of the rock is derived from the decomposition of 
the particles of calcareous matter which once filled its cavities, 
while the blackness of the mass arises from the altered state of 
the iron included in the chlorite, forming an integral part, or the 
colouring matter of the rock itself. 
The sides of the pass, as its name denotes, rise to a great 
height on both sides; but they afforded us, so far as we were 
able to inspect them, no variety differing in any important degree 
from the slates and rocks of what we have termed the base of 
Moel Shabod, and the slaty cleavage runs in the same direction; 
viz. north-east and south-west. ‘The same rocks, as far as we 
were competent to judge of them from below, or from the nume- 
rous and often very large fallen masses, were the same to the 
very summit, with the exception of a few varieties, which we 
shall presently notice. We perceived many instances on the 
large scale of their taking the rounded form already noticed. 
These separate masses are often traversed by neatly horizontal 
lines, or by small cavities which, from their nearly ovate figures, 
might be assumed to have been derived from their having once 
been occupied by substances of similar form: they do not, how- 
ever commonly present any impression ; but the surface of seve- 
ral fallen fragments of considerable dimension that had sepa- 
rated along these lines, were covered by cavities altogether 
resembling each other, and several of them presented on one 
mass the casts of ribbed shells. These lines of cavities, it will 
be observed, being nearly horizontal and parallel to each other, 
were about, but not quite, at right angles to the slaty cleavage 
of the rock. We succeeded in detaching the impression of a 
shell lying in one of these nearly horizontal lines, from a rock 
perfectly resembling that variety, at the base of Moel Shabod, 
which has most the appearance of homogeniety. 
Among the fallen masses in this pass, we found some resem- 
bling flinty slate, and possessing the same character, though 
harder than those occurring around and upon Moel Shabod. We 
also found masses greatly resembling a steatitic substance, of a 
yellowish-white colour, and so hard as to yield with difficulty to 
