All at once he changed his course, and darted around a small rocky promontory. The tide was rising and he had to step into the water to make the turn.
"After him!" yelled Frank again.
The two brothers made the turn, and just far enough behind the man to see him dart into the black entrance of a small cave. It was one they had looked into, but into which they had not penetrated far.
"Now we've got him!" yelled Andy. "There's no way out of that! Come on, Frank!"
Together the two brothers entered the dark cavern. The change from the glaring sunlight on the sands to intense gloom made them pause for a moment, and they heard from somewhere in the blackness of the rear a sinister chuckle.
"He's in here," declared Andy. "We have him now."
The two pressed forward resolutely in the darkness. Of what lay before them—the danger from a desperate man and the danger of the cavern they knew not—they only resolved to end the mystery if possible.
CHAPTER XX
THE RISING TIDE
"Where are you, Frank?" called Andy.