1822.] Geology of Snowdon, and the surrounding Country. 323 
as to yield readily to the knife, sometimes even to the nail, but 
occasionally so intermixed with siliceous matter, as scarcely to 
receive any impression from the knife : in the latter case it is 
generally of a greyish colour ; in the former greenish, the colour- 
ing matter being in our estimation chlorite, which often is so 
arranged as to impart to the rock a slaty structure; and we 
possess the most unquestionable proofs that the impressions of 
shells occurring within 10 feet of the very summit of Snowdon 
are in a rock of this nature, of which doubtless the occasional 
fineness of the grain, and its generally slaty structure, has given 
rise to the notion that these impressions occur in a greywacké 
slate. The imbedded substances, when the rock possesses a 
porphyritic aspect, appear to be contemporaneous nodules harder 
than the rock itself, quartz, more rarely felspar, frequently car- 
bonate oflime, the three latter being generally crystalline, tran- 
sparent, and very minute ; chlorite, however, is so commonly an 
ingredient of all the steatitic rocks, either in very minute parti- 
cles, or in layers, that it may be said to form almost an essential 
ingredient, and is commonly present in so large a proportion as 
to impart its greenish colour to the mass. 
Chlorite, however, sometimes prevails so greatly, as almost to 
exclude the other ingredients, and then appears in the form of 
chlorite slate, which occurs near the summit of Snowdon, within 
perhaps 20 feet of the steatitic rock containing the impressions of 
shells ; and is even interstratified with layers of the same nature. 
The two extremes, therefore, appear to us to be steatite and 
chlorite ; but each of these two substances is often so modified 
by combining with the other in different proportions, and proba- 
bly also by the intimate dispersion through the mass of siliceous 
matter, and by its occasionally imbedding small nodules of cal- 
careous spar, felspar, and grains of quartz (the latter in one 
instance prevailing to the almost total exclusion of all the rest), 
that the rocks assume a great variety of aspect, and even appear 
to differ so greatly, that nothing short of an inspection of the 
whole series, or seeing them either interstratified or passing into 
each other, as we have mostly seen them, would suftice to pro- 
duce a conviction of their actual and even intimate connexion, 
The slates forming so considerable a proportion of the sur- 
face of this country, are also of very different aspects ; varying 
from nearly pure steatite of a greenish colour, soft enough to 
it to the pressure of the nail, through still harder varieties, 
th externally and by transmitted light of the same colour, to 
consist chiefly of silex and alumine, but included a small proportion of lime. Ona 
repetition of his experiments for the purpose of ascertaining whether an alkali is or is not 
present in a rock most nearly resembling steatite, no alkali was detected, but the pre- 
sence of a very minute trace of magnesia was indicated, These rocks, therefore, 
differ from steatite and potstone in containing little or no magnesia; but as it is essen- 
tial to adopt some name for the sake of reference in the following pages, we shall in 
speaking of them use the term steatite, which may serve until some more appropriate 
esignation shall be given to them, 
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