"Isn't likely that they fell into the hands of Tuareg raiders, is it?" asked Mr. Holton. "Tishmak said not," Joe's father returned. "He thinks that the Tuaregs around here are friendly and peaceful." A little farther on they passed several large slabs of rock that were covered with ancient inscriptions and drawings. Dr. Kirshner would surely throw a fit of delight if he could see them. But, thought Bob, perhaps he had already come to them and was searching for more, quite forgetful of the necessity to get back to camp. But no, this could not be, for Joe would realize that necessity if the scientist did not. At noon they stopped for lunch under a large crag that resembled an inverted hook. There was plenty of water in their containers, giving each the opportunity to satisfy his thirst. There was not much conversation during that rest, for they were all busy with their thoughts. And those thoughts were anything but cheerful. But they refused to give up hope so soon, for another half-hour might see the two lost ones back with their friends. Scarcely ten minutes were spent in the shade of the rock. Even then Mr. Lewis felt that that time had been wasted. "Something might have happened to them during that time," he said, greatly worried. Mr. Holton laid a hand on his friend's shoulder. "Snap out of it, Ben, old man," he begged. "I'm willing to bet that they are safe and well. And I firmly believe we will find them before another day will pass." Mr. Lewis did resolve to cheer up and be hopeful; but still he possessed a haunting fear that perhaps tragedy had come upon them. The sun blazed very hot that afternoon, scorching the already baked soil. This did not hinder the progress of the searchers, however, who trudged constantly on. Suddenly, as they passed over a wide valley strewn with rocks and dotted with cracks and ravines, Bob stopped and looked about expectantly. The others had also caught a sound. The next moment the explorers were seized with fear, for ten or twelve evil-looking Arabs darted out on dromedaries and rushed toward the explorers. "What's this!" cried Mr. Holton, as one of the men caught hold of his camel. There was no chance to use their guns. The Arabs were on the lookout for any treachery. Suddenly Bob saw a chance and, drawing the reins of his dromedary, he dashed away at full speed for a short canyon between the rocks. CHAPTER XXIV A Dangerous Undertaking Urging his dromedary on to the utmost, the youth was carried over the narrow valley at a pace that ate up the distance surprisingly. In fact, never had he traveled so rapidly on camelback. When he was several hundred feet away, he glanced back, to see that two of the Arabs were pursuing him desperately. At the start he had put a good distance between himself and his enemies, but now the latter were gaining swiftly. They were more familiar with the country and knew better how to command their camels to move fast.