e.

"It sounded like something falling," was Andy's opinion. "Let's go
forward and see what it was."

They had not gone forward more than a dozen steps before they were
halted by the sound of a voice--the voice of the mysterious man.

"Maybe you'll take a warning next time!" he sneered. "I think you've
followed me once too often. This is the end."

They could hear him hastening away. Then came silence.

"What did he mean?" asked Andy.

"I don't know," replied his brother. "Let's look."

Andy was in the lead. Slowly he advanced, flashing his electric light.
Then he came to a halt.

"What's the matter?" asked Frank.

"I can't go any farther. The passage ends here in a solid rock."

"Then we'll have to go back. I thought he was fooling us. He wanted
to get us in some side chamber, so he could make his escape from the
entrance. Hurry back."

They fairly ran to the other end of the passageway, retracing their
steps. This time Frank was ahead. Suddenly he came to a halt.

"Well, why don't you go on?" asked Andy.

"I can't. There's a big rock here."

"A rock? There wasn't any there when we came in."

"I know it, but it's fallen down since. The passage is closed."

"Closed!" gasped Andy. "Then I know what happened. That was the noise
we heard. That man toppled this rock down to trap us here. We're
caught, Frank! Caught!"

For a moment the older brother did not answer. Then he replied:

"It does looks so. But we'll try to shove this stone out of the way.
Come on, lend a hand."

Together the boys pushed and shoved. But all to no purpose. Flashing
their lights on the obstruction, they saw that it had fallen down in a
wedged-shaped place, dove-tailing itself in so that no power short of
dynamite could loosen it. The hopelessness of moving it struck them at
once.

"The other end!" cried Frank. "We must try to get out the other way!"

Back they raced along the passage, slipping stumbling on the wet, rocky
floor. But it only to come face to face with a solid wall of rock.

"No use trying to get through there," said Andy. "We must try to move
the big rock."

"We can't," spoke Frank. "I think----" But he never finished that
sentence. Instead focused his light down on the stone floor of passage
in the cave. A thin stream of water trickling along it.

"Look! Look!" whispered Andy.

"Yes," answered his brother in a low voice. "The tide is rising. It's
running into the cave,