“No,” said Frank, “let me go.  He saved my life, and I am the lightest.  Please let me go!”

“Yes,” said Mr. March, “let Frank go.  It is much better that he should.”

Mr. Elmer reluctantly consented that Frank should take his place, and the rope was fastened around the boy’s body, under his arms, having first been wound with saddle blankets so that it should not cut him.  Taking a lighted torch in one hand and some fresh splinters in the other, he slipped over the log which they had placed along the edge, so that the rope should not be cut by the rocks, and was gently lowered by the three anxious men into the awful blackness.

Thirty feet of the rope had disappeared, when it suddenly sagged to the opposite side of the hole, and at the same instant came the signal for them to pull up.

As Frank came again to the surface the lower half of his body was dripping wet, and his face was ghastly pale.

“He isn’t there,” he said; “but there is a stream of running water so strong that, when you let me into it, I was nearly swept away under the arch.  It flows in that direction,” he added, pointing to the south.

CHAPTER XVI.

Buried in an underground river.

When Mark felt himself flying from his horse’s back through the air, he of course expected to strike heavily on the ground, and nerved himself for the shock.  To his amazement, instead of striking on solid earth he fell into a mass of shrubbery that supported him for a moment, and then gave way.  He grasped wildly at the bushes; but they were torn from his hands, and he felt himself going down, down, down, and in another instant was plunged deep into water that closed over his head.  He came to the surface, stunned and gasping, only to find himself borne rapidly along by a swift current.  He did not for a moment realize the full horror of his situation, and with the natural instinct of a swimmer struck out vigorously.

He had taken but a few strokes when his hand hit a projecting rock, to which he instinctively clung, arresting his further progress.  To his surprise, on letting his body sink, his feet touched bottom, and he stood in water not much more than waist deep, but which swept against him with almost irresistible force.

His first impulse was to scream, “Frank! oh, Frank!” but only a dull echo mocked him, and he received no reply but the rush and gurgle of the water as it hurried past.

Then in an instant he comprehended what had happened.  He had been flung into a “sink hole,” and was now buried in the channel of one of those mysterious underground rivers of which Mr. March had told them a few nights before.  That was at home, where he was surrounded by his own loving parents and friends.  Should he ever see them again?  No; he was buried alive.

Buried alive! he, Mark Elmer?  No—­it couldn’t be.  It must be a dreadful dream, a nightmare; and he laughed hysterically to think how improbable it would all seem when he awoke.