nd
if so, we must explore it also. Of course, I am most interested in the
outlet to the south."

A circuit was made until they reached the outlet to the south, which
Harry had discovered when the light on the ledge disappeared. The water
throughout the cavern within the chamber was not over eight feet deep,
and at the outlet to the south he could not touch bottom with the
twelve-foot pole they carried. This outlet was contracted, and, judging
from the width of the boat, could not be more than eight feet across,
but it gradually widened, and the waters became shallower as they
advanced.

George, who was in the bow, held up his hand as a warning. "Stop!" was
all he said. All peered forward. The lights threw their beams forwardly
through a broadening channel, beyond which appeared to be white forms
ranged along the opposite wall.

"What depth have you, Harry?" asked the Professor, without seeming at
all concerned.

"About five feet."

"Move straight ahead, until I tell you to stop."

The pole was thrust down and the boat moved forwardly fully fifty feet
before a halt was called.

"I think we are now in the middle of this chamber. Before exploring it
let us make a thorough examination of its characteristics."

"Look at those wonderful icicles hanging from the roof!" George gazed on
them with wonder and admiration. Harry, on the other hand, with the
utilitarian idea in his mind, inquired: "Why couldn't all that chalk be
utilized for making plaster?"

"That product is used in the arts, but it costs too much to transport it
from the places where it is found in its natural state, as science has
found a much cheaper way of producing it from limestone."

"Are all these rocks limestone?"

"Beyond question. Only a few of the caves so far found are in any other
formation than limestone."

"What kind of cave are those?"

"Where they have been produced by volcanic action. There the walls are
of volcanic rock."

"Why is it that these underground channels are formed in this way?"

"They are formed by the erosive action of the water wearing out the
softer portions of the rock beneath a harder roof or wall. This action
is brought about by carbonic acid acting on the rock and producing what
is called carbonate of lime, and the stalactites and stalagmites found
in all these caverns are of that material."

"What is the difference between the two names you have just mentioned?"

"Stalactite means trickling or dropping,