Critical Factor

Pentong, excited for the first time in his life, raced northward. There was no need to grope or feel his way; this close to the great earthquake zone there were always minor tremors, and their echoes from the dense basalt below and the emptiness above reached him almost constantly. The treacherous sandstone strata, which beguiled the lazy traveler with the ease of penetration they offered and then led him up to the zones of death, were easy to spot; Pentong actually used them now, for seeing was so good that he could leave them with plenty of time to seek the safer levels below whenever they started to slope.
The worst of his journey was behind. The narrow bridge of livable rock which led to the strange land he had found had been recrossed in safety, in spite of the terrifying and deceptive manner in which temblors from the earthquake zone far to the north were trapped, magnified, and echoed from its sides. Now he could see for many days’ travel all about him, and as far as he could see the land was good.
Not as good as that he had visited, of course. This was the land he had known all his life, where food was just hard enough to find to make life interesting; where for ages past counting other, less fortunate, races from the far, far north had sought to break in and kill that they might inherit its plenty; where pools of magma shifted just rapidly enough to trap the unwary between impenetrable basalt and glowing death; where, if Pentong was right in what he believed of his discovery, regions now too close to the zones of death might be made accessible and provide food and living space for unguessable generations to come.
He dreamt of this possibility constantly as he moved. No trace of his passage marked the rock behind him, for none of it was edible; but he hardly thought of food for himself. Speed was his prime concern, and to achieve it he traveled as close as he dared to the upper zones.
The nearest settlement was more than five thousand miles north, he knew; his memory held a sharp picture of the tortuous path he had followed from it, and he retraced that path now. It led him far to the east, where the earth tremors were faint and travel slowed by the poor vision; then back, at a much lower level, to the northwest, where the principal delay was the denser rock. Five hundred miles short of his goal he had to stop, to examine carefully the region of magma pools through which he had passed on his way south. The precise path he had followed could not now be used; it was blocked in several places by molten rock which had forced its way between strata and heated the otherwise habitable stone above and below to an unbearable degree. But other paths existed; and slowly and carefully Pentong wormed his way between the pools, sometimes retreating the way he had come, sometimes going almost straight away from his goal, but gradually working north and downward until the last of the dangerous pockets of fluid lay behind him. Then he could hasten once more; and at last he reached the bed of carbonate rock, a mile thick and more than thirty thousand square miles in area, which had been deposited on the floor of an ancient sea some hundreds of millions of years before and was now safely surrounded and capped by harder layers which shielded its inhabitants from filtering oxygen. This was the city—not the one where Pentong had been born, but the farthest south of all the dwelling centers of his people, and the one to which the more adventurous spirits of the race tended to gravitate. The cities to the northwest and northeast, under the Bering and Icelandic bridges, held danger, of course; they bore the brunt of the endless defense against the savage tribes from beyond the bridges. Still, that danger was known and almost routine; it was the unknown parts of the world that spelled adventure. Pentong, he was sure, had proved himself the most adventurous so far; and he was also sure that he had done more.