a few miles when the man paused on the flat top of a high hill, which on the side away from them sloped steeply into a deep gorge at the foot of which ran a small stream. They watched him narrowly.

With great care he got the four mules together, standing side by side. He himself took up a position directly in front of them and almost touching the animals’ heads.

A moment later man and mules sank together, apparently into the earth and disappeared!

They could hardly believe their eyes! Surely the man must have gone down the reverse slope of the hill. But they were confident that he had not moved.

They hurried to the spot. Not a sign of any living thing was to be seen! The mystery was profound.

While they stood gazing at one another in speechless amazement, the Mohawk, which they had not perceived above them, dropped vertically downwards and landed a few yards away. Dick sprang out.

“Did you see?” he gasped. “The man and mules went down into some sort of pit. But where was it?”

The flat top of the hill was broken into a series of narrow cracks; apparently the rock of which it was composed was of volcanic origin. They examined it closely, but they could discover nothing which offered a solution of the mystery.

Dick described closely what he had seen from the sky. It agreed with what the others had observed. The man had got the mules together, and all had sunk slowly downward. Dick had seen the black mouth of the pit for a few moments and a blaze of light. Then the pit had disappeared, and the ground resumed its normal appearance.

“We shall have to camp here to-night,” said Dick. “We must get to the bottom of this. We shall have to take turns to watch. In the meantime we had better have a look round.”

Having closely examined the top of the hill, they turned to the deep gorge and descended to the bottom. The stream, they found, issued from the hill itself, flowing out from a low tunnel high enough to admit the passage of a man. From it also issued a cloud of mist which spread over the bottom of the little valley in a thick blanket which completely concealed the surface of the ground from anyone at the top of the hill.

But still more remarkable was that the bed of the little stream was deeply covered with what appeared to be 

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