Jimmie called from the tent, and Ned and Frank moved on down the declivity toward the lake.

It was still early evening, and the moon was low down in the east, so the valley where the lake lay was not touched by its light. Indeed, the plateau where the boys were would have been in the shadow of the mountain only for the dropping of the shoulder of the divide.

In half an hour the two boys, after several slides which were anything but pleasant, gained the beach. The campfire was now dead, and the locality was still save for the voice of a night bird and the occasional splash of a leaping fish. The mouth of the cavern loomed like a dark patch on the lower bulk of the mountain.

Making as little noise as possible, Ned and Frank crept into the cavern, advancing by the sense of feeling until they came to the very end before turning on one of the electric flashlights. The round eye of the flame showed a long, narrow, tunnel-like tube running directly east, under the mountain. The door of rock was as the boys had left it earlier in the day.

Ned examined that portion of the rock which had swung out into the first chamber with considerable care, as the story of the swinging stone had interested him greatly. All along the top, up to the center, he found the checks of a stone-chisel. Exactly in the middle an elevation of an inch fitted into a round cavity in the upper rock. At the bottom the same conditions were discovered.

“Rather a clever job,” Ned said, “but I don’t see how it was ever done.”

“This door,” Frank said, “is not exactly like the remainder of the wall in grain, so it must have been brought here from some other locality. Of course there was a hole between these two chambers, or the second one would never have been found. It would be easy enough to fit the stone door in by grooving out from the lower cavity and sliding the under pivot in.”

“Sure,” Ned replied, getting down to examine the lower part of the door more closely, “and that is just what was done. Then the groove was filled with concrete. Pretty classy work here!”

“And now the question is this,” Frank went on, “what was the door fitted for? Why did the men who found the cave desire privacy? Is there gold in there? Have the men who have been setting fire to the forests established a home here? Is this the hiding place of a band of outlaws? You see there are lots of questions to ask about the two caverns,” Frank added, with an uneasy laugh.

Ned closed the stone door and turned on both electric flashlights, making the place light as day where they stood. The inner cavern was as bare as the outer one save for dead leaves and grass which lay in heaps on the stone floor, and for half a dozen rough benches which were piled in one corner. At the farther end hung a gaudy curtain, once handsome, but now sadly spotted with mildew because of dampness.

“Here’s the inner chamber,” laughed Frank, drawing the curtain aside. “And it looks like it was the private office of the bunch, too,” he added, as he turned the light about the walls.

There was a desk in the third cavern, a swivel chair, a small case of books, and a rusty safe, which looked as though it had not been opened for years. A current of fresh air came from the rear, and a small opening was soon discovered.

“That doubtless leads to some cañon not far away,” Ned said. “Makes a pretty decent place of it, eh?”

“Good enough for any person to hide in,” replied Frank. “Now,” he added, “tell me what you think of it. Who cut this cavern, and who brought the furniture here? I’ll admit that my thinker is not working.”

“Nature made the caverns,” Ned replied. “There is what geologists call a fault in the rock here. Owing to volcanic action, doubtless, the strata shifted, probably thousands of years ago, and when the seam appeared the broken pieces fell apart. These chambers show the width of the seam. There undoubtedly was a great earthquake at the time, and the lake below might have been dredged out at that time.”

“Of course,” Frank said, “I might have known that! Now, here’s another question: How far does this seam extend under the Rocky Mountains? If it passes beyond these three chambers, why not make a fourth room for ourselves so as to be on the spot when the men who make headquarters of the place come back?”

“That may be a good thing to do,” Ned admitted, “but, still, I would not like to be the one to lie in wait here. Suppose we try to learn something of the character of the people who come here? They seem to sleep on dry leaves and eat off benches. Rather tough bunch, I take it. Perhaps we have struck Uncle Sam’s enemies the first thing!”

Keeping their lights on, and working as silently as possible, always with an eye to the outer cavern, the boys made a careful search of the inner chamber. The desk was not fastened, and a cupboard afterward discovered in a niche was open also. There were dishes in the cupboard and writing materials in the desk.

At the very bottom of the desk drawer Ned came upon a surprise.

“Not so tough as I supposed,” he said, turning to Frank. “Here’s a typewriter ribbon. The sort of people who set fire to forests and hold up trains are hardly in the typewriter class. What do you make of it?”

“Well,” Frank said, with a chuckle, “if you’ll tell me what the inhabitants of this place want of typewriter ribbons I’ll tell you why they bring great tins of opium here. It seems that we have struck something more important than forest fires.”

CHAPTER IV.—THE AEROPLANE IN DANGER.

A strong wind came out of the Western Sea at ten o’clock that night and swept the lofty plateau as a woman might have swept it with a new broom. Ned and Frank, pursuing their investigations in the cavern, knew nothing of what was going on at the camp, but Jack and Pat were not long in ignorance of the danger of the situation.

With the first strong rush of wind the boys were on their feet, steadying the aeroplane, driving stakes wherever the nature of the ground permitted, and running bracing cords. The shelter tents went down instantly and were blown against the rocks of the east, where they waved canvas arms in the tearing breeze like sheeted ghosts.

The black clouds which swarmed up from the valley brought no rain, but fitful flashes of lightning and deep-toned thunder made a threatening sky. The roaring of the swirling trees in the cañon and on the slopes came up to the ears of the boys like the boom of a strong surf.

After persistent efforts the boys succeeded in bracing the aeroplane so that there was little danger of its being swept away, though they still remained with their backs to the wind, holding on. As time passed, they crept close together in order that the situation might be discussed.

“Lucky thing we remained here,” Pat said, tugging with all his might to steady the monster machine against a particularly vicious dash of wind.

“It would have gone sure, if we hadn’t,” Jack screamed back. “I wish Ned and Frank would come and help. My back is creaking like a shaft that needs oiling with the strain on it.”

“A little help wouldn’t go amiss,” Pat admitted, shouting at the top of his lungs in order that he might be heard above the whistling of the storm.

“I wonder if we’ll ever be able to put the tents up again?” Jack shouted. “They are flapping and snapping like musketry out there on the rocks. I hope they won’t blow away entirely.”

Pat gazed anxiously in the