416 Messrs. W. Phillips and §. Woods on the [Dec. 
colour, and manifestly steatitic ; the weathered surfaces show 
white specks, some of which yield to, while others resist the 
knife; the former consist of calcareous spar, the latter, judging 
by their fracture in the interior of the rock, and which, there- 
fore, had not suffered any change by exposure, consists of stea- 
tite mechanically mixed with siliceous matter, since they 
yesemble the larger masses included in the steatitic rock at the 
base of Moel Shabod; and being of a somewhat greenish 
colour, are not always distinguishable on the surface. To these 
succeeded a very imperfectly slaty rock, of a greenish colour, 
the weathered surfaces almost perfectly white—an appearance 
not very uncommon to the softest varieties of the more com- 
pletely steatitic rocks. A still softer variety afterwards occurred, 
apparently consisting of a mechanical mixture of chlorite and 
steatite, and containing white specks of carbonate of lime. 
Rocks of the same description continued to prevail up the ascent 
of Cwm Clogwim, the upper part of which is not less than 2500 
feet above the sea. The most remarkable of these consist 
chiefly of slaty chlorite, yielding, however, a brisk effervescence 
by the application of acid, though carbonate of lime is not appa- 
rent even through a glass. 
As we approached the summit of Snowdon, keeping close to 
the ridge called Widdfa, we found it to consist entirely of 
several of the abovementioned rocks and slates repeated without 
any appearance of order in their recurrence, and with their 
cleavage plane (being all more or less slaty) nearly vertical, and 
in the direction of about NEand SW. In some of the porphy- 
ritic varieties, the slaty substance, when the softest of the two, 
had decomposed, leaving opaque white masses resembling those 
of the same rocks above Cwm Clogwin, projecting above the 
surface, while in others the imbedded substance (calcareous 
spar) had passed away, leaving the slaty rock vesicular. Car- 
bonate of lime enters into the composition of the slaty part of 
both these varieties since they both effervesce in acid, but no 
quartz is perceptible in them. The substance of the rock is 
manifestly slaty chlorite. A variety of this description occurs 
interstratified on the very summit, with an apparently homoge- 
neous and very soft, yet brittle, slate of a greenish colour. 
It has been known for some time that the impressions of a 
peculiar shell occur in considerable abundance within a few feet 
of the summit of Snowdon, and the rock enclosing them has 
been termed greywacke. We are decidedly of opinion that not 
a single rock occurs on Snowdon, nor as far as our observation 
extended, near that mountain that is af all allied to greywacke, 
unless the blue slates already noticed as prevailing up the ascent 
from Llyn Cweilyn, and others bearing the same characters, can 
be so considered, unaccompanied, as they certainly are, by 
ereywacke itself, and often interstratified with rock of a dect- 
dedly steatitic, or of a chloritic base. These shells oceur-in 
