is
now, by a flat isthmus, across which, upon the falling of the tide, the
ancient Cornish miners used to carry over their tin in carts. Had the
relative levels of sea and land been those of the old coast-line at the
time, St. Michael's Mount, instead of being accessible at low ebb would
have been separated from the shore by a strait from three to five
fathoms in depth. It would not have been then as now, as described in
the verse of Carew,--

"Both land and island twice a day."

But even the incidental notice of Diodorus Siculus represents very
inadequately the antiquity of the existing coast-line. Some of its
caves, hollowed in hard rock in the line of faults and shifts by the
attrition of the surf, are more than a hundred feet in depth; and it
must have required many centuries to excavate tough trap or rigid gneiss
to a depth so considerable, by a process so slow. And yet, however long
the sea may have stood against the present coast-line, it must have
stood for a considerably longer period against the ancient one. The
latter presents generally marks of greater attrition than the modern
line, and its wave-hollowed caves are of a depth considerably more
profound. In determining, on an extensive tract of coast, the average
profundity of both classes of caverns from a considerable number of
each, I ascertained that the proportional average depth of the modern to
the ancient is as two to three. For every two centuries, then, during
which the waves have been scooping out the caves of the present
coast-line, they must have been engaged for three centuries in scooping
out those of the old one. But we know _historically_, that for at least
twenty centuries the sea has been toiling in these modern caves; and who
shall dare affirm that it has not been toiling in them for at least ten
centuries more? But if the sea has stood for but even two thousand six
hundred years against the present coast-line (and no geologist would
dare fix his estimate lower), then must it have stood against the old
line, ere it could have excavated caves one third deeper, three thousand
nine hundred years. And both periods united (six thousand five hundred
years) more than exhaust the Hebrew chronology. Yet what a mere
beginning of geologic history does not the epoch of the old coast-line
form! It is but a mere starting point from the recent period. Not a
single shell seems to have become extinct during the last six thousand
five hundred years! The