n their backs patches of newer beds, the rest of which has long
vanished; and in their rise they have hurled back to the eastward,
and set upright, what is now the whole western flank of Snowdon, a
mass of rock which was then several times as thick as it is now.

The force which thus tortured them was probably exerted by the great
mass of volcanic Quartz-porphyry, which rises from under them to the
north-west, crossing the end of the lower lake of the Llanberris; and
indeed the shifts and convulsions which have taken place between them
and the Menai Straits are so vast that they can only be estimated by
looking at them on the section which may be found at the end of
Professor Ramsay's "Geological Survey of North Wales." But anyone
who will study that section, and use (as with the map) a little
imagination and common sense, will see that between the heat of that
Porphyry, which must have been poured out as a fluid mass as hot,
probably, as melted iron, and the pressure of it below, and of the
Silurian beds above, the Cambrian mud-strata of Llanberris and
Penrhyn quarries must have suffered enough to change them into
something very different from mud, and, therefore, probably, into
what they are now--namely, slate.

And now, at last, we have got to the slates on the roof, and may
disport ourselves over them--like the cats.

Look at any piece of slate. All know that slate splits or cleaves
freely, in one direction only, into flat layers. Now any one would
suppose at first sight, and fairly enough, that the flat surface--the
"plane of cleavage"--was also the plane of bedding. In simpler
English we should say--The mud which has hardened into the slate was
laid down horizontally; and therefore each slate is one of the little
horizontal beds of it, perhaps just what was laid down in a single
tide. We should have a right to do so, because that would be true of
most sedimentary rocks. But it would not be true of slate. The
plane of bedding in slate has nothing to do with the plane of
cleavage. Or, more plainly, the mud of which the slate is made may
have been deposited at the sea-bottom at any angle to the plane of
cleavage. We may sometimes see the lines of the true bedding--the
lines which were actually horizontal when the mud was laid down--in
bits of slate, and find them sometimes perpendicular to, sometimes
inclined to, and sometimes again coinciding with the plane of
cleavage,