th,
wiggles around a little like a river with its tributaries, though nothing
like so regularly, with here and there a wider place, and----"

"Here and there," contributed his chum, "a well to a lower level."

"Yes. You see, the water that wears a cave out of the softer layers of
rock seeps in along the fissures of the surface rock, and at first they
make subterranean rivers. Where you find these big springs in the
hillsides, they may be the outlets of these underground waterways."

"I get that, all right," said Ted.

"Well, then, sometimes these Stygian streams----"

"Keep it up, Professor!" Ted clapped him on the shoulder.

"Huh!--These rivers wear away the soft limestone layer,--if it is this
kind of a cave,--'till they come to the harder sandstone. Then the first
chance they find to get through the sandstone,--perhaps through a crack
made by an earthquake or something,--they go down and wear away a deeper
level. Mammoth Cave is on five levels. That leaves the upper galleries
dry. Now the one we were on was dry except for the moisture that is
always seeping into a cave, but I suspect now we're on a level with the
river, it's so muddy, and we'll find it somewhere."

"Then we'll find it somewhere!" brightened Ted. "And we can follow it.
That's the plan of action!" and he jumped to his feet.

"We'll follow it if we can. Thunder! I wish we had a boat."

"So long as you're wishing, why don't you wish for a fat steak with
onions?"

"It has been some time since we ate." Ace tightened his belt. "Must be
getting late in the day! Let's run!" And run they did, till they began
slipping on a muddy slope.

They had to place each foot with care now, and their progress was slow.
At the same time their candles were nearly gone. "Now let's put out all
but one," suggested Ted. "Just burn one at a time. What _would_ we do
without any light?" But Ace did not know the answer.

What of Pedro, meantime? At that particular instant he had just tried to
make his get-away, with the result that three drawn daggers were being
flourished threateningly and most unhealthily near his heart. He had
overheard enough evidence to convict all three of the Mexicans, thanks to
his knowledge of the parent language, but as the desperadoes pushed
farther and farther into the labyrinth, he gathered that they would come
out a good safe distance from where they had entered,--probably on the
other side of the ridge. Had he known the Ranger's whereabou