1822.] Geology of Snowdon, and the surrounding Country. 335 
masses we cannot doubt from their form were found interstrati- 
fied with the other rocks of this neighbourhood, and from the 
trifling appearance of disintegration on many of them, we sus- 
ected them to have been quarried for the purpose to which they 
hain been applied. We did not, however, succeed in finding 
the quarry, owing, perhaps, to the still very unfavourable state of 
the weather. We have since been informed by Mr. Jos. Woods, 
that he has seen the quarries in question in that neighbourhood, 
and that these masses are found as we had suspected. Rolled 
masses of this rock, both porphyritic and otherwise, prevail 
every where in the low ground and valleys, and have been much 
used as a building stone. 
On our return to Capel Curig, we passed the foot of Trefan, a 
mountain of considerable elevation, whose remarkably serrated 
outline, and rugged scarp attracted our attention, and induced 
the wish of ascending it for the purpose of becoming acquainted 
with the nature of its constituent rocks. But the extreme wet- 
ness of the weather deprived us of the power of seeing much 
that would have been highly interesting. Mr. Dawson after- 
wards presented us with a specimen of arock having the appear- 
ance of a conglomerate, and resembling some already described 
as appearing near the base of Moel Shabod, and which he 
informed us prevailed on the ascent of Trefan. The same gen- 
tleman showed us a small fragment of one of the two remarkable 
upright pillars on its summit, which resembled some of the small 
grained rocks, hereafter to be noticed, as occurring near the 
summit of Snowdon. 
(To be continued.) 
Articcie If. 
On Siliceaus Petrifactions imbedded in Calearecus Rock. 
By the Rey. J. J. Conybeare. 
(To the Editor of the Anals of Philosophy.) 
DEAR SIR, Bath Easton, Oct, 2, 1822. 
Tue perusal of the note occurring at p. xii. of the preface to 
the “ Geology of England and Wales,” has induced me to sub- 
mit to a rude examination such specimens as | possess of silice- 
ous petrifactions imbedded in calcareous rock. I wish that my 
results were more calculated to throw light upon their produc- 
tion, and that they may induce some one better qualified to pay 
the subject that attention which it demands. 
1. Coral in separate Branches protruding from the Surface of 
Masses (apparently either Nodules or weathered Portions) of dark 
blue Limestone (Mendip Hills)—The exposed portion of coral 
