The sandhills here were higher than they had been before, and there were openings between them as if passages led into the interior valleys, so that Caius supposed that here in storms or in flood-tides the waves might enter into the heart of the dune.
They had not travelled far beyond the first and nearest wreck, when the monotony of their journey was broken by a sudden strange excitement which seized on them all, and which Caius, although he felt it, did not at once understand.
The pony was jerked back by the reins which O'Shea held, then turned staggering inland, and lashed forward by the whip, used for the first time that day.

Caius, jerked against the side of the cart, lifted up a bruised head, gazing in wonder to see nothing in the path; but he saw that the boy had sprung lightly from the cart, and was standing higher up on the sand, his whole attitude betraying alarm as he gazed searchingly at the ground.
In a moment the pony reared and plunged, and then uttered a cry almost human in its fear.

Then came the sensation of sinking, sinking with the very earth itself.

O'Shea had jumped from the cart and cut the traces.
Caius was springing out, and felt his spring guided by a hand upon his arm.

He could not have believed that the boy had so much strength, yet, with a motion too quick for explaining words, he was guided to a certain part of the sand, pushed aside like a child to be safe, while the boy with his next agile movement tugged at the portmanteaus that contained the medical stores, and flung them at Caius' feet.
It was a quicksand.