ly relics
below, seemed to stun him.

"Have you a reason for wanting to know how high up we are?" asked George
when he had partially recovered.

"You wanted to know a little while ago how the smoke in the cavern might
affect them. Haven't you noticed a perceptible movement in the
atmosphere since we entered the chamber?"

The boys started and stared at him. Could it be that the cave had an
outlet in the hills?

"Was that the reason you suggested we should make a circuit around the
chamber after we entered it?"

"Yes; and I know where the outlet is."

"And does that explain why the pirates made their home at this end?"

"Undoubtedly; and what will be still more interesting information is,
that the opening is within sight of the cataract."

Could anything be more exciting than this information?

"I now see the reason why you always wanted to come back to the cave.
Did you suspect this when we first entered the cave?"

"No; but I had an idea we should find this after we made our second
trip."

"What did you see?"

"Nothing but what Harry brought to me."

"What was that?" both exclaimed, eagerly.

"The slab of carbonate which Harry brought me for the marking tablet,
and on which we made the chart of the cave."

"What did you find on it?"

"If you will recall, I brought it with me. It is now in the boat." Harry
dashed down to the boat and brought it back, turning it over and over on
the way.

The slab referred to was about two inches thick, a foot long, and
probably ten inches wide, a little irregular in formation.

"When we returned home that evening, after the trip referred to, I took
the slab and transferred the chart we had made to a board. In doing so,
I noticed that the lime had been chipped away from one side, but that
did not cause me to make any investigation at the time.

"Some days afterwards I again took it up, and could see plainly through
the carbonate what appeared to be the shadows of some characters, and it
at once occurred to me that, owing to the sunlight and the comparatively
dry atmosphere in which it had been kept after its removal, that the
lime would turn or change its color, but the lime on this background did
not change in the same degree where the characters had been placed, and
when we get into the sunlight you will be able to see just what I saw."

Looking at the slab, there was nothing to indicate any characters
imprinted in it.

"Where is the opening, Professor?"