burned as if fire had lately issued
from the earth. Every now and then they came, too, upon soft patches of
mud firm enough to walk over and like india-rubber beneath their feet;
but it was nervous work, and they crossed with care, feeling, as they
did, a curious vibration going on beneath their feet.

Then came an exceedingly rugged descent of quite a precipitous nature,
but lovely in the extreme, so clothed was it with tropic verdure, though
this was more beautiful to the eye than to the feet, for it often
concealed treacherous crevices between blocks of scoria, and ugly cracks
and rifts, some of which were dangerous, while others were awful from
their depth and the low, hissing, murmuring sounds which came from their
inmost recesses.

At last the descent became so precipitous that they were brought to a
stand-still and all progress seemed to be at an end, till, searching
about, Mark and Billy Widgeon came upon a broad gash in the mountain
side at the bottom of which there seemed to be a long slope of the
smooth, hard-surfaced mud apparently running downward toward the spot
they sought.

The captain declared the descent practicable with care, and Mark took
the lead, going down with plenty of agility, and closely followed by the
little sailor.

At the end of a quarter of an hour they were all on the stony brink of
what seemed to be a mud-stream which at some time had flowed down from
out of a huge yawning chasm high up above their heads, and perfectly
inaccessible from where they stood. According to all appearances, this
mud in a thin state must have come down in a perfect cataract till it
filled up the space beneath the chasm, which resembled a huge basin, as
level as so much water, and when this had become full the stream had
begun to form, and down this mud-stream they proposed to go, though how
far it extended and would help them on their way experience alone could
show.

They stood just at the edge of the pool to find that a walk upon its
surface would be dangerous in the extreme, for though the top was
elastic a stick was easily driven through, with the result that a jet of
steam rushed out with a noise like that of a railway whistle, but the
surface of the stream on being tested proved firmer, and they began to
descend.

Again the same sense of insecurity was felt, the india-rubber-like film
giving way easily and springing up again, while the old muttering and
murmuring noises thrilled beneath their fe