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.
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