When my reader has studied awhile the confusion--for it is a true
confusion--of the different beds, he will ask, or at least have a
right to ask, what known process of nature can have produced it? How
have these various volcanic rocks, which he sees marked as Felspathic
Traps, Quartz Porphyries, Greenstones, and so forth, got intermingled
with beds which he is told to believe are volcanic ashes, and those
again with fossil-bearing Silurian beds and Cambrian slates, which he
is told to believe were deposited under water? And his puzzle will
not be lessened when he is told that, in some cases, as in that of
the summit of Snowdon, these very volcanic ashes contain fossil
shells.
The best answer I can give is to ask him to use his imagination, or
his common sense; and to picture to himself what must go on in the
case of a submarine eruption, such as broke out off the coast of
Iceland in 1783 and 1830, off the Azores in 1811, and in our day in
more than one spot in the Pacific Ocean.
A main bore or vent--or more than one--opens itself between the
bottom of the sea and the nether fires. From each rushes an enormous
jet of high-pressure steam and other gases, which boils up through
the sea, and forms a cloud above; that cloud descends again in heavy
rain, and gives out often true lightning from its under side.
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