When they emerged from the Great Road, the Great Desert lay shimmering before them. Under the sun, standing directly overhead in a cloudless sky, the irregular floor of sand stretching illimitably in three directions like gently-rolling waves of the sea arrested in motion, seemed radiating heat waves like the top of a stove. In the fourth direction, at their backs, stretched away on either hand the Great Mountain Wall. For the first time by daylight they had a good look at it, and involuntarily both Jack and Frank drew in their breaths at the sight. Steep, precipitous, verdureless, the mountains rose in great masses of rock directly out of the sand, just as the Pillars of Hercules, guarding the Strait of Gibraltar, rise out of the sea at the northwestern extremity of the African continent. The sand was like a sea dashing vainly against these gigantic masses of rock. One mountain overlapped another, so that only narrow, unscalable clefts broke the face of this tremendous bulwark. Truly, it was a Mountain Wall. Ahead this wall stretched to the horizon, and, looking over their shoulders, the boys saw no end in sight behind them. A full two thousand feet towered the serrated summits, jagged and sawtoothed, forming grotesque shapes that resembled crumbling turrets and, in some instances, crouching animals of gigantic proportions. The previous evening, before the light failed, and while they still were hours away, they had been able to see that beyond this wall lay a tumbled mass of mountain tops rising higher and higher to a lofty summit far to the south, which Mr. Hampton had estimated at 14,000 or 15,000 feet in height. Apparently, then, the mountain country was of considerable extent, with many interior valleys and plateaus. Somewhere behind that Mountain Wall and in the heart of that great upland country lay mysterious Athensi, while through the rich valley and plateaus were scattered the dwellings of the peasants comprising the last of a prehistoric white race whence had sprung, if Professor Souchard was correct, all mankind. But to these reflections Jack and Frank, unable because they rode close to the foot of the Mountain Wall and could not see the peaks of the country behind it, gave only passing reflection. Almost immediately on debouching into the desert they had discerned far in the distance a number of tiny figures drawing away from them to the west. These undoubtedly were the horsemen of the rear guard of Captain Amanassar’s rebel troop. “Come on, Jack,” yelled Frank, “I’ll race you for them.” Jack yelled agreement, and away went the two boys, jouncing and bumping in the awkward camel saddles, their animals eating up the ground, while beside them galloped the four young Athensians. The boys had all they could do to hold on, but once or twice managed to steal a glance over their shoulders which assured them Mr. Hampton, Ali and the Arabs were close in the rear.