I have been told, for instance, that that wonderful little
blue Glas Llyn, under the highest cliff of Snowdon, is the old crater
of the mountain; and I have heard people insist that a similar lake,
of almost equal grandeur, in the south side of Cader Idris, is a
crater likewise.
But the fact is not so. Any one acquainted with recent craters would
see at once that Glas Llyn is not an ancient one; and I am not
surprised to find the Government geologists declaring that the Llyn
on Cader Idris is not one either. The fact is, that the crater, or
rather the place where the crater has been, in ancient volcanoes of
this kind, is probably now covered by one of the innumerable bosses
of lava.
For, as an eruption ceases, the melted lava cools in the vents, and
hardens; usually into lava infinitely harder than the ash-cone round
it; and this, when the ash-cone is washed off, remains as the highest
part of the hill, as in the Mont Dore and the Cantal in France, and
in several extinct volcanoes in the Antilles. Of course the lava
must have been poured out, and the ashes blown out from some vents or
other, connected with the nether world of fire; probably from many
successive vents. For in volcanoes, when one vent is choked, another
is wont to open at some fresh point of least resistance among the
overlying rocks.
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