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 Incas didn’t do that, I hope,” said Harry, turning to me.

“Hardly,” I answered; and in my absorbing interest in the phenomenon before me I half forgot my pain.

I moved to the edge of one of the walls extending at right angles to the passage, but there was little to be made of it.  It was of soft limestone, and most probably the portion that had disappeared was granite, carried away by the force of its own weight.

“We are like to be buried,” I observed, returning to Harry and Desiree.  “Though for that matter, even that can hardly frighten us now.”

“For my part,” said Harry, with a curious gravity beneath the apparent lightness of his words, “I have always admired the death of Porthos.  Let it come, and welcome.”

“Are we to go further?” put in Desiree.

Just as Harry opened his mouth to reply a more decisive answer came from another source.  The rock that had fallen, obstructing the path of the Incas, must have left an opening that Harry had missed; or they had removed it—­what matter?

In some way they had forced a passage, for as Desiree spoke a dozen spears whistled through the air past our heads and we looked up to see a swarm of Incas climbing and tumbling down the face of a boulder over which we had passed to reach our resting-place.

I have said that we had halted in a level, unbroken stretch that still led some distance ahead of us.  At its farther end could be seen a group of rocks and boulders completely choking the lane, Beyond, other rocks arose to a still greater height—­the way appeared to be impassable.

But there was no time for deliberation or the weighing of chances, and we turned and made for the pile of rocks, with the Incas rushing after us.

There Desiree and I halted in despair, but with a great oath Harry brushed us aside and leaped upon a rock higher than his head with incredible agility.  Then, lying flat on his face and extending his arms downward over the edge, he pulled first Desiree, then myself, up after him.  The whole performance had occupied a scant two seconds, and, waiting only to pick up the three spears he had thrown up the sloping surface of the rock to another yet higher and steeper.