a volcano! After the first astonishment was over, Mark made a close
examination of the place.

The mound, or barrier of lava and scoriæ that composed the outer wall of
this crater, was almost mathematically circular. Its inner precipice was
in most places absolutely perpendicular, though overhanging in a few;
there being but two or three spots where an active man could descend in
safety. The area within might contain a hundred acres while the wall
preserved a very even height of about sixty feet, falling a little below
this at the leeward side, where there existed one narrow hole, or
passage, on a level with the bottom of the crater; a sort of gateway, by
which to enter and quit the cavity. This passage had no doubt been
formed by the exit of lava, which centuries ago had doubtless broken
through at this point, and contributed to form the visible reef beyond.
The height of this hole was some twenty feet, having an arch above it,
and its width may have been thirty. When Mark got to it, which he did by
descending the wall of the crater, not without risk to his neck, he
found the surface of the crater very even and unbroken, with the
exception of its having a slight descent from its eastern to its western
side; or from the side opposite to the outlet, or gateway, to the
gateway itself. This inclination Mark fancied was owing to the
circumstance that the water of the ocean had formerly entered at the
hole, in uncommonly high tides and tempests, and washed the ashes which
had once formed the bottom of the crater, towards the remote parts of
the plain. These ashes had been converted by time into a soft, or
friable rock, composing a stone that is called tufa. If there had ever
been a cone in the crater, as was probably the case, it had totally
disappeared under the action of time and the wear of the seasons. Rock,
however, the bed of the crater could scarcely be yet considered, though
it had a crust which bore the weight of a man very readily, in nearly