The usual fire was necessary to save them.

They were coated solidly with ice, and the two men kept them on the run around the fire, sweating and thawing, so close that they were singed by the flames.
At another time Spitz went through, dragging the whole team after him up to Buck, who strained backward with all his strength, his fore paws on the slippery edge and the ice quivering and snapping all around.

But behind him was Dave, likewise straining backward, and behind the sled was Francois, pulling till his tendons cracked.
Again, the rim ice broke away before and behind, and there was no escape except up the cliff.

Perrault scaled it by a miracle, while Francois prayed for just that miracle; and with every thong and sled lashing and the last bit of harness rove into a long rope, the dogs were hoisted, one by one, to the cliff crest.

Francois came up last, after the sled and load.