Andy and Roebach needed no urging to the attack. Nor did the boys. They disembarked carefully and made a detour so as to get at the rear of the herd. The sea lion is not a very sagacious beast. Jack and Mark were on either side of the old hunter and were moving upon the herd with considerable circumspection, and all had about come to a place where the rifles could be used effectively, when Jack Darrow spied something that brought a cry to his lips. Fortunately both the hunter and Mr. Roebach fired the next instant and two of the sea lions were hit. The remainder of the herd slid over the ice-cliff and flopped away at good speed toward a break in the ice through which they could get into the water. But Jack began to dance and shout, and Mark was too surprised to even fire at the herd. "What under the sun is the matter with you, chum?" exclaimed Mark, with some asperity. "You're as bad as Washington White." "Maybe I'm worse," bawled the cheerful Jack. "You scared off them sea lions, boy," admonished Andy Sudds. "We only got two of them." "Don't care if I did," replied Jack. "See yonder!" The others followed the direction of his pointing arm with their gaze. Off beyond the headlands at the mouth of the river rose a column of thick black smoke. It was as big a smoke as though some great forge or factory was working overtime in that direction. "Hurrah!" cried Mark, re-echoing his chum's delight. The entire party was delighted. Yet not knowing who the people were who made the smoke, nor under what circumstances they would find them, the dead sea lions were packed aboard the sleds before they continued their way down the river. "That smoke lies a good way beyond the mouth of the river," said Phineas Roebach. "I believe it is on the sea." "A vessel afire?" proposed Mark. "It's a fire on a vessel," said the professor, suddenly. "I believe that is the smoke of the trying-out works on a whaler." "You've hit it, Professor," agreed Andy Sudds. "It's a whaler for sure. There's more than _you_, Phineas, hunting for oil up in these regions." "A whaling ship on this island in the air," murmured Jack. "What will they do with the whale oil? They will never get back to San Francisco again." "We do not know that," said the scientist, gravely. The last few miles, during which they could not see beyond the high ice-shod banks of the estuary, were traversed slowly enough. They all grew anxious to know what the column of black smoke meant. Finally they came to the open mouth of this branch of the river. The sight they beheld almost stunned them. Instead of an ocean, rolling up in great surges upon the beach on either hand, they beheld a vast sink through which the partly ice-bound river crawled as far as the eye could see. They knew that this was the old bed of the Arctic Ocean; but the waters of that cold sea had receded and left little but ice-bound pools here and there.