1822.] Geology of Snowdon, and the surrounding Country. 411 
slaty rocks, and in slates which are interstratified ; the slates 
are of a greenish colour, the cleavage being less perfect than: 
that of ordinary slates, and thin portions of them are green by 
transmitted light: they sometimes include whitish particles, 
which yield to the knife, but not to acid, and which we, there- 
fore, conclude to be harder portions of the same substance as 
the rock itself, and of contemporaneous origin; and in this 
respect resembling the slaty rocks with which they are interstra- 
tified, and which also contain the impressions of shells. These 
rocks are sometimes porphyritic from the same cause as the 
slates, and being extremely soft, yield readily to the knife, and 
consist chiefly of steatite intermixed, and generally coloured by 
specks and layers of chlorite, which sometimes has been decom- 
posed, leaving cavities: the steatite, however, is often of a 
yellowish colour, and somewhat translucent. That the softness 
of these rocks is not owing to the progress of decomposition is 
manifest from the inspection of the interior of vast blocks, occur- 
ring not on this mountain alone, and which have by accident or. 
design been cleft asunder. 
The rocks above enumerated were observed gn situ as they” 
appeared on the surface, or rather on the very ddge of the pre- 
cipices along the ascent of the mountain; and we conceive both 
from their examination individually, and from the circumstances 
of their connexion, that they are to be considered as varieties of 
the same rock: most of them pass into each cther, some even 
in the course of a very few inches NE and SW. Thus what 
appears in one place as an almost homogeneous slate of a pretty 
close texture is sometimes so altered in its character in a short 
distance as to include multitudes of small white particles, whick, 
when they effervesce in muriatic acid, we consider to be car- 
bonate of lime, but when they do not, they appear to be of con- 
temporaneous formation with the rock itself, and to consist of 
steatite intimately mixed with siliceous matter ; and the charac- 
ter of the rock alters in becoming closer grained and less slaty 
in structure. It is certain, however, that in other places the 
interstratification of large masses of the rocks already described 
with slates is very manifest ; for owing to the greater hardness 
of the rocks, and their less liability to decomposition than the 
slates, they frequently form ridges which mse uncovered by 
herbage above the slates on either side of them, and are seen 
projecting beyond the slates down the precipices of the Widdfa, 
and of that over Cwm Clogwin, while the indentations made by 
the decomposition of the slates (those which appear the most 
perfectly homogeneous yield soonest to atmospherical action) 
are covered by herbage, as is apparent every where from the 
per? beginning of the ascent from the banks of Llyn Cwellyn. 
n our descent beneath Crib Coch, and thence to Llanberris 
Pass, we observed no other rocks than such as have already 
