recently melted lava. In many places it was still hot, and the water, they found, was nearly boiling. The first traces of this were found at the mouth of the tunnel from which the stream emerged, and for hundreds of yards the molten rock could be traced, as though it had poured from the tunnel and flowed down the bed of the brook.

Wood and water were available in abundance, and soon they had pitched their camp, near enough to the top of the mysterious hill to enable them to watch it closely and yet well concealed so that if the man reappeared they would have no difficulty in escaping observation.

The first watch fell to Yvette, and with a revolver ready for instant use, she prepared to spend a couple of lonely hours on the edge of the hill. The camp was but a quarter of a mile away so that a shot would bring her speedy help at any time.

A brilliant moon lit up the country for miles.

There was no trace of any living thing. Everything was still and silent.

Yvette had been on watch about an hour when she became aware that the air was full of a dull murmur of sound. She listened intently. There was no mistake about it. A dull throbbing noise was distinctly discernible.

She walked round the flat top of the hill, looking keenly in every direction and trying to locate the position from which the mysterious sound was coming. But it was in vain.

Glancing into the gorge, she saw a strange and terrible phenomenon. The course of the little brook was traced in a dull fiery glow. Clouds of steam were rising thickly into the night air; she could plainly hear the sharp hiss of water on something hot.

She ran swiftly down the hill. At the bottom she paused on the edge of the stream. The water had disappeared and in its place ran a river of molten rock! Through her boots she felt the heat of the ground.

Returning to the top of the hill she waited for Dick, who was now almost due to relieve her. In a few moments he appeared and listened in amazement as she gasped out her story.

The dull, throbbing noise was still audible.

“Machinery,” said Dick laconically, “but where?”

Suddenly he flung himself on his face, and pressed his ear close to the ground.

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