rock some three hundred feet high, dividing, some
distance from the top into three sharp pinnacles. On the surface of the
middle spire could be seen a small black dot. The Indians were dancing
with excitement, and the boys themselves felt a thrill as they realized
that they were nearing the climax of a great mystery.

"That looks like a great river bed, in which the water had dried up,"
remarked Rand, "I never saw anything like it before."

"Bed of an old glacier," said the guide, who had come up. "Lots of 'em in
this country."

"That explains it, then," said Jack, excitedly.

"Explains what?" inquired Dick.

"How they got up there," replied Jack. "Don't you see? This valley was
full of ice once nearly to the tops of those rocks, and when it came down
and melted off, the bodies of the mammoths dropped out, and the natives
gathered the tusks and stored them in the cave which they could easily
reach with the glacier so near the top. Then the snow gave out somewhere
in the mountains and the glacier gradually pushed its way out and melted,
leaving the cave high and dry."

"All right for you, Jack," said Gerald. "Begorra, you've had that story
already written, I see. But it looks like the real goods."

"I've read of these things before," replied Jack.

"That's about what happened," commented the guide. "Some geological sharps
who were up here last year explained one of these rocky holes the same
way."

The pack horses were now brought up to the top of the ridge and unloaded,
as they could not very easily be taken down the valley slope. With the
greatest care the plane was removed from the two pack animals, and with
ropes lowered on its own wheels down the gravelly slope. The motor and
other machinery was slid down upon skids cut from the forest and placed
along the bank. At the bottom, the Scouts set to work putting the machine
together.

"Ah," said the guide, with the air of a great discoverer, "I see what yer
scheme is now. Ye're goin' up in that arrerplane, and see if ye can git a
peek in that hole up there."

"Better than that," replied Gerald. "We're going to get up and get into
that hole."

Delighted at finding they were nearing the goal of their hopes with so few
obstacles, the Scouts worked cheerfully and earnestly upon the
reassembling of the plane, and by noon had replaced the motor and tested
every stay, brace and control. Then, after a dinner of caribou meat and
coffee, they wheeled the plane over