Stern's observation of the rising flood proved correct. By whatever theory it might or might not be explained, the fact was positive that now the water there below them was rising fast, and that inside of half an hour at the outside the torrent would engulf their ledge.
It seemed as though there must be some vast, rhythmic ebb and flux in the unsounded abysses that yawned beneath them, some incalculable regurgitation of the sea, which periodically spewed forth a part, at least, of the enormous torrent that for hours poured into that titanic gulf.
And it was upon this flux, stormy and wild and full of seething whirlpools, that Allan Stern and the girl now built their only possible hope of salvation and of life.
"Come, we must be at work!" he told her, as together they peered over the edge and now beheld the weltering flood creeping up, up along the thunderous plunge of the waterfall till it was within no more than a hundred feet of their shelter.
As the depth of the fall decreased the spray-drive lessened, and now, with the full coming of day, some reflection of the golden morning sky crept through the spray. Yet neither to right nor left could they see shore or anything save that long, swift, sliding wall of brine, foam-tossed and terrible.
"To work!" said he again. "If we're going to save ourselves out of this inferno we've got to make some kind of preparation. We can't just swim and trust to luck. We shall have to malice float of some sort or other, I think."
"Yes, but what with?" asked she.
"With what remains of the yawl!"
And even as he spoke he led the way to the crevice where the splintered boards and the torn sail had been wedged fast.
"A slim hope, I know," he admitted, "but it's all we've got now."
Driven home as the wreckage was by the terrific impact of the blow, Stern had a man's work cut out for him to get it clear; but his was as the strength of ten, and before half an hour had passed he had, with the girl's help, freed all the planks and laid them out along the rock-shelf, the most sheltered spot of the ledge.
Another hour later the planks had been lashed into a rough sort of float with what cordage remained and with platted strips of the mat sail.
"It's not half big enough to hold us up altogether," judged the man, "but if we merely use it to keep our heads out of water it will serve, and it's got the merit of being unsinkable, anyhow. God knows how long we may have to be in the water, little girl. But whatever comes we've got to face it. There's no other chance at all!"