t it was some sound made by the volcano. We'll have to try
something else."

Both were silent for a few moments, immersed in bitter thoughts that
were as black as the darkness that surrounded them.

"Can you ever forgive me, Ruth, for having gotten you into such a trap
as this?" he burst out suddenly.

"You didn't get me in it," protested Ruth. "I came in of my own
accord."

"I don't mean that," explained Drew. "But you tried to persuade me not
to enter the cave in the first place, and if I'd only had sense enough
to listen to you; we'd both of us be out in the sunlight at this
minute. Headstrong fool that I was!" he ended in an agony of self
condemnation.

"Now don't blame yourself a bit for that, Allen," said Ruth earnestly.
"You only did what you thought you ought to do, and ninety-nine times
out of a hundred no harm would have come of it."

"And it was our luck to strike the hundredth time," replied Drew
bitterly.

"Besides," said Ruth with a trifle of hesitation, "I think I'd have
been a little disappointed at the time if you had done as I asked. I'd
have felt that perhaps in your secret heart you did it apparently to
please me, but really because you were glad enough not to have to take
any chances of what you might meet in here."

Drew was somewhat puzzled at this bit of feminine psychology, but he
gathered some comfort from it, and this was perhaps after all the
result that Ruth was seeking.

"Do you notice, Allen, how fresh the air seems to be in here?" she
asked.

"I've been wondering at that," he answered. "To tell the truth my
worst fear has been that it would get too close and foul for us to
breathe. But it seems to be just as sweet now as it was at the
beginning."

"What do you suppose is the reason?"

"It must be that the cave is a little larger than it seems to be. It
seemed to be getting bigger and bigger as I went further into it. If
that is so, it accounts for the fact that the air supply has not yet
begun to be vitiated."

"But mayn't there be any other reason?" she asked.

"I can't think of any other," he answered. Then as a thought suddenly
struck him, he jumped as though he had been shot.

"Why didn't I think of that before?" he fairly shouted. "There may be
another entrance!"

CHAPTER XXIV

THE ALARM

Unaware of the possible tragedy that was being developed within a few
hundred yards of them, Tyke and Captain Hamilton had kept on digging in
the excavati