I said; "limestone."

"Well, you ought to know how limestone ridges are honeycombed with
water-formed caverns. We have several examples at home. If this
subterranean river came bubbling up from somewhere in the interior and
the rock were granite, I should expect it to be hot."

"And it's quite cold, sir," said Cross.

"Oh, no, just pleasantly cool. I don't think there's a doubt about its
having its source higher up in the mountains; but whether it has dived
down for a few hundred yards or a few miles we can only know by
exploring."

"Well, Cross," I said to the carpenter, "will this be fun enough for
you?"

"Splendid, sir," said the man enthusiastically. "I never had a treat
like this."

"Master Nat," whispered Pete, "am I to come too?"

"Of course," I said. "Tuck up your trousers as high as you can."

"But suppose we have to swim, sir?"

"Look here, Pete," I said, "you don't want to come."

"No, sir. Can't help it, sir, but I never could a-bear the dark."

"Then I'll ask my uncle to let you stop behind."

"What!" cried the poor fellow fiercely, "leave me behind, and you go?
That you just won't, sir. I'd go if it was twice as dark."

I saw him set his teeth, and then, as my uncle gave the word, he climbed
up to a verdant cleft with Cross to cut four stout bamboos about six
feet long to act as walking-staves.

"We must always be ready to feel our way and try the depth," said Uncle
Dick; "and avoid any holes. If it grows deeper as we go on and there is
no bare rock at the sides, of course we must return."

A few minutes later our guns were slung across our backs, the loads
taken up, and, each armed with a staff, we made our start--Cross, as he
held the lanthorn, asking leave to lead the way.

"We shan't be able to do it, Master Nat," whispered Pete, as we followed
in turn, Pete last, for it was very hard work, the barrels of our guns
scraping again and again against the roof during the first twenty yards
or so; but Pete had hardly uttered the above words before I saw Cross
raise the lanthorn higher. Then my uncle began to walk erect, and
directly after I found on raising my staff that I could not touch the
roof, while a sharp whistle uttered by our lanthorn-bearer was echoed
from far on high.

"Plenty of room upwards, sir," cried Cross.

"Yes," said my uncle.

"Ugh! what a horrid place, Master Nat!" whispered Pete, who kept as
close to me as he could. "Do mind, sir."

"Mind what?" I s