adjoining sea (taken chiefly from Captain Austin's survey and some old
charts), will show the nature of this difficulty.

If, as seems probable, the basaltic streams were originally prolonged with
nearly their present inclination, they must, as shown by the dotted line in
the section, once have extended at least to a point, now covered by the sea
to a depth of nearly thirty fathoms: but I have every reason to believe
they extended considerably further, for the inclination of the streams is
less near the coast than further inland. It should also be observed, that
other sections on the coast of this island would have given far more
striking results, but I had not the exact measurements; thus, on the
windward side, the cliffs are about two thousand feet in height and the
cut-off lava streams very gently inclined, and the bottom of the sea has
nearly a similar slope all round the island. How, then, has all the hard
basaltic rock, which once extended beneath the surface of the sea, been
worn away? According to Captain Austin, the bottom is uneven and rocky only
to that very small distance from the beach within which the depth is from
five to six fathoms; outside this line, to a depth of about one hundred
fathoms, the bottom is smooth, gently inclined, and formed of mud and sand;
outside the one hundred fathoms, it plunges suddenly into unfathomable
depths, as is so very commonly the case on all coasts where sediment is
accumulating. At greater depths than the five or six fathoms, it seems
impossible, under existing circumstances, that the sea can both have worn
away hard rock, in parts to a thickness of at least 150 feet, and have
deposited a smooth bed of fine sediment. Now, if we had any reason to
suppose that St. Helena had, during a long period, gone on slowly
subsiding, every difficulty would be removed: for looking at the diagram,
and imagining a fresh amount of subsidence, we can see that the waves would
then act on the coast-cliffs with fresh and unimpaired vigour, whilst the
rocky ledge near the beach would be carried down to that depth, at which