As they got higher, the ascent became more and more difficult, for the flanks of the mountain were almost perpendicular, and it required the utmost care to keep them from falling. Clouds of scoriae and ashes would whirl round them repeatedly, threatening them with asphyxia, or torrents of lava would bar their passage. In parts where these torrents ran horizontally, the outside had become hardened; while underneath was the boiling lava, and every step the travellers took had first to be tested with the iron-tipped staff to avoid being suddenly plunged into the scalding liquid. At intervals large fragments of red-hot rock were thrown up from the crater, and burst in the air like bomb-shells, scattering the debris to enormous distances in all directions. Hatteras, however, climbed up the steepest ascents with surprising agility, disdaining the help of his staff. He arrived before long at a circular rock, a sort of plateau about ten feet wide. A river of boiling lava surrounded it, except in one part, where it forked away to a higher rock, leaving a narrow passage, through which Hatteras fearlessly passed. Here he stopped, and his companions managed to rejoin him. He seemed to be measuring with his eye the distance he had yet to get over. Horizontally, he was not more than two hundred yards from the top of the crater, but vertically he had nearly three times that distance to traverse. The ascent had occupied three hours already. Hatteras showed no signs of fatigue, while the others were almost spent. The summit of the volcano appeared inaccessible, and the Doctor determined at any price to prevent Hatteras from attempting to proceed. He tried gentle means first, but the captain's excitement was fast becoming delirium. During their ascent, symptoms of insanity had become more and more marked, and no one could be surprised who knew anything of his previous history. "Hatteras," said the Doctor, "it is enough! we cannot go further!" "Stop, then," he replied, in a strangely altered voice; "I am going higher." "No, it is useless; you are at the Pole already." "No, no! higher, higher!" "My friend, do you know who is speaking to you? It is I, Doctor Clawbonny." "Higher, higher!" repeated the madman. "Very well, we shall not allow it--that is all." He had hardly uttered the words before Hatteras, by a superhuman effort, sprang over the boiling lava, and was beyond the reach of his companions. A cry of horror burst from every lip, for they thought the poor captain must have perished in that fiery gulf; but there he was safe on the other side, accompanied by his faithful Duk, who would not leave him. He speedily disappeared behind a curtain of smoke, and they heard his voice growing fainter in the distance, shouting-- "To the north! to the north! to the top of Mount Hatteras! Remember Mount Hatteras!"