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 every part of it.  Once or twice Mark broke through, as one would fall through rotten ice, when he found his shoes covered with a light dust that much resembled ashes.  In other places he broke this crust on purpose, always finding beneath it a considerable depth of ashes, mingled with some shells, and a few small stones.

That the water sometimes flowed into this crater was evident by a considerable deposit of salt, which marked the limits of the latest of these floods.  This salt had probably prevented vegetation.  The water, however, never could have entered from the sea, had not the lava which originally made the outlet left a sort of channel that was lower than the surface of the outer rocks.  It might be nearer to the real character of the phenomenon were we to say, that the lava which had broken through the barrier at this point, and tumbled into the sea, had not quite filled the channel which it rather found than formed, when it ceased to flow.  Cooling in that form, an irregular crevice was left, through which the element no doubt still occasionally entered, when the adjacent ocean got a sufficient elevation.  Mark observed that, from some cause or other, the birds avoided the crater.  It really seemed to him that their instincts warned them of the dangers that had once environed the place, and that, to use the language of sailors, “they gave it a wide berth,” in consequence.  Whatever may have been the cause, such was the fact; few even flying over it, though they were to be seen in hundreds, in the air all round it.

Chapter V.

    “The king’s son have I landed by himself;
    Whom I left cooling of the air with sighs
    In an odd angle of the isle, and sitting,
    His arms in this sad knot.”

    Tempest.

Having completed this first examination of the crater, Mark and Bob next picked their way again to the summit of its wall, and took their seats directly over the arch.  Here they enjoyed as good a look-out as the little island afforded, not only of its own surface, but of the surrounding ocean.  Mark now began to comprehend the character of the singular geological formation, into the midst of which the Rancocus had been led, as it might almost be by the hand of Providence itself.  He was at that moment seated on the topmost pinnacle of a submarine mountain of volcanic origin—­submarine as to all its elevations, heights and spaces, with the exception of the crater where he had just taken his stand, and the little bit of visible and venerable