“It is only a matter of time now,” I said to Desiree, and she nodded.

Still we went forward.  The land had carried us straight away from the cavern, without a turn.  Its walls were the roughest I had seen, and often a boulder which lay across our path presented a serrated face that looked as though it had but just been broken from the wall above.  Still the stone was comparatively soft—­time had not yet worked its leveling finger on the surfaces that surrounded us.

We were standing on one of these boulders when Harry came running toward us.

“They’re stopped,” he cried gleefully, “at least for a little.  A piece of rock as big as a house gently slid from above onto their precious heads.  It may have blocked them off completely.”

We hurried forward then; Harry helped Desiree, while I painfully brought up the rear.  At every few steps they were forced to halt and wait for me, though I did my utmost to keep up with them.  Harry had taken my spear that I might have both hands to help me over the rocks.

Climbing, sliding, jumping, we left the Incas behind; no sound came from the rear.  I began to think that they had really been completely shut off, and several times opened my mouth to call to Harry to ask him if it would not be safe to halt; for every movement I made was torture.  But each time I choked back the cry; he thought it was necessary to go on and I followed.

This lasted 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.