sted I know not how long; I was staggering and reeling forward
like a drunken man, so little aware of what I was doing that when Harry
and Desiree finally stopped at the beginning of a level, unbroken
stretch in the lane, I stumbled directly against them before I knew
they had halted.

"Go on!" I gasped, struggling to my feet in a mania.

Harry stooped over to assist me and set me with my back resting against
the wall. Desiree supported herself near by, scarcely able to stand.

"We can go no farther," said Harry. "If they come--"

As he spoke I became aware of a curious movement in the wall
opposite--a movement as of the wall itself. At first I thought it a
delusion produced by my disordered brain, but when I saw Desiree's
astonished gaze following mine, and heard Harry's cry of wonder as he
turned and saw it also, I knew the thing was real.

A great portion of the wall, the entire side of the passage for a
length of a hundred feet or more, was sliding slowly downward.
Glancing above I saw a space of several feet where the rock had
departed from its bed. The only noise audible was a low, grating sound
like the slow grinding of a gigantic millstone.

None of us moved--if there were danger we would seem to have welcomed
it. Suddenly the great mass of rock appeared to halt in its downward
movement and hang as though suspended; then with a sudden jerk it
seemed to free itself, swaying ponderously toward us; and the next
moment it had fallen straight down into some abyss below, thundering,
tumbling, sliding with terrific velocity.

There was a deafening roar under our feet, the ground rocked as from an
earthquake, and it seemed as though the wall against which we stood was
about to fall in upon us. Dust and fragments of rock filled the air on
every side, and one huge boulder, detached from the roof above, came
tumbling at our feet, missing us by inches.

We were completely stunned by the cataclysm, but in a moment Harry had
recovered and run to the edge of the chasm opposite thus suddenly
formed. Desiree and I followed.

There was nothing to be seen save the blackness of space. Immediately
before us was an apparently bottomless abyss, black and terrifying; the
side descended straight down from our feet. Looking across we could
see dimly a wall some distance away, smooth and with a faint whiteness.
On either side of us other walls extended to meet the farther wall,
smooth and polished as glass.

"The Inca