Ned shouted in alarm and rushed up after him, Sir John and the doctor
next, for there was no reply to the shouts, and as just then a puff of
smoke suddenly shot into the air, a horrible dread assailed the little
group. But when they reached the edge they saw that their alarm was
needless, for Jack had dropped into a sitting position upon the soft
ashes, and was gazing down into a great cup-like depression about
half-a-mile across, and gradually dipping down till the centre of the
hollow was about five hundred feet below the top.

"Not much to see, Ned," said Jack as the man joined him. "That must be
where the bright glow comes from at night."

He pointed down over the dark silvery grey waste, dotted with stones of
all sizes, to where a pool lay on one side, apparently of water, for a
shimmering light played over it, and a faint mist was rising slowly into
the air.

"Couldn't come from water, sir," said Ned. "I didn't expect to see a
pond up here; but I suppose it's hot, and that's steam."

"Oh yes, that's hot enough," said the doctor, who was panting with his
exertions. "Liquid fire, eh, Jack?"

"Wouldn't it be molten metal of some kind, father?" cried the boy.

"No, my lad, it is molten stone--rock. Lava."

"But it puzzles me," cried Jack, "how stone can melt. You said
something to me one day about a flux."

"Yes, of course. People who smelt metals found that out long enough
ago, and it is the same with making glass. If you expose some minerals
separately to great heat they merely become powder; but if you combine
them--say flinty sand with soda or potash--they run together and become
like molten metal. I believe if ironstone and limestone are mixed, the
ironstone becomes fluid, so that it can be cast like a metal--in fact
becomes the metal itself."

"Then that pool down there, if emptied out, would run like the volcanic
glass we have found below?"

"Most likely."

"Let's go down this slope so as to see the pool from nearer."

"Rather a risky proceeding, my boy," said Sir John; "suppose we were to
break through."

"Break through? Why, you don't think it is hollow under here?"

"I should rather believe that there was a stony crust hardened by
cooling, and that a very short distance beneath us the rocks are all
molten."

"But all these great stones lying about don't break through. Let's go a
little way down."

"Don't be rash then. Will you come, Instow?"

"Oh yes, if it's safe. Let's g