1822.] Geology of Snowdon, and the surrounding Country. 413 
for the decomposition taking place in so unusual a form, and 
this, as we conceive, we are enabled to do from the observation 
of a fact, equally remarkable and unexpected. 
In a country in which every appearance of actual stratification 
is parallel to the cleavage of the slates, we certainly should not 
expect to find the very rocks thus interstratified exhibiting cha- 
racters which are to be accounted for only on the supposition of 
their having been deposited in the opposite direction, namely, 
nearly, but not quite, horizontally. In descending the Widdfa, 
and near the summit of the Lluwydd, we sawemany rocks in 
situ, and even in very large masses, which exhibited nearly hori- 
zontal and alternate projections and depressions on their sides, 
and we found that these uniformly consisted of layers of varieties 
of the same rock, differing both in colour, and sometimes even 
in composition, the slaty cleavage being uniform and nearly at 
right angles to the direction of the layers. Those which were 
of the lightest colour, and were, therefore, judged to contain 
the least proportion of iron, being least subject to decay, pro- 
truded, while the ends of those which were darkest, having 
suffered the most by exposure, formed the indentations. These 
effects commonly take place on the large scale, but we were so 
fortunate as to find more than one instance in which a cabinet 
specimen completely illustrates the fact, each thus forming in 
itself a sort of epitome of the rocks of this region. We shall, 
therefore, attempt to describe one of the specimens in question, 
first noticing the fact, that these rocks do not possess a cleavage 
parallel to the direction of the variously coloured layers. 
The base of the whole mass, which is about four inches long, 
and three wide, is manifestly steatite. It yields every where to 
the knife, affording a white powder. It consists of about 20 
bands or layers in a direction not quite at right angles to the 
cleavage plane, varying from a yellowish-grey colour (when it is 
considerably compact) to a dark-green, which, however, is much 
heightened by the addition of moisture, and which arises from 
the intermixture of abundance of chlorite (it is then more com- 
pletely slaty) ; the two extreme bands are of this nature, but one 
of them is vesicular from the decomposition of the calcareous 
spar once imbedded in it, and it still effervesces abundantly ; the 
other extreme approaches the character of ordinary slate: 
between these are others, but very thin, of the same nature lying 
between others of a yellowish-grey colour with little or no inter- 
mixture of chlorite. Another specimen affords the opportunity 
of observing the very different effects of atmospherical action on 
the differently-coloured bands. This is much softer than the 
former in every part, and consists of, perhaps, 50 grey and dark- 
green layers varying from the tenth to the fiftieth of an inch in 
thickness: the dark-green have decomposed, leaving the grey 
ones protruding nearly half an inch. 
It has been observed, that the cleavage plane of the slates in 
