The next moment he was all excitement, having evidently come across something on the side of the rock. "Drawings!" he cried animatedly, pointing to the wall about him. "Prehistoric drawings of--of elephants!" CHAPTER XX The Horror of Thirst "Elephants?" asked Bob, almost bursting out in laughter. "Come on," suggested Joe, moving slowly down the side of the fissure. "Let's have a look at the strange drawings." The youths slid to the bottom, where Dr. Kirshner stood staring at the wall. Bob nodded. "Drawings of elephants, all right," he said, his eyes on the etched rock. "And look how plain they are." The archæologist took out his notebook and copied the sketches as best he could. Then he turned to the youths. "Here is proof that the desert was not always a desert," he said, his eyes becoming bright with interest. "Thousands of years ago this region was green with tropical vegetation, like the dense forests of East Africa. It was probably inhabited by tribes of people much different from the Arabs and Tuaregs who now live here. Then came a gradual dry spell, and in time the luxurious growth gave way to a hot desert of sand and rocks." "Those drawings of elephants, then, were made while this region was covered with forests?" questioned Joe, becoming as interested as the scientist. The latter nodded. "Elephants and other wild game probably roamed about here in great numbers," he explained. After one last look at the strange sketches, the explorers began the task of climbing up the side of the ravine. It was not easy to pull themselves up out of the steep crevice, but the rocky walls were solid, not even threatening to give way. Then followed an hour of exploration about the top of the cliff, during which time the archæologist came upon the remains of many other ancient drawings and inscriptions. By the time that they were ready to begin the descent of the cliff, he had filled his notebook. "But when we get to the Ahaggars we'll undoubtedly find many more," he said, slowly leading the way down. After what seemed a long time, they came to the bottom of the precipice and lost no time in getting back to camp. "Have any luck?" asked Mr. Holton, looking up with interest as the three explorers moved toward the tent. "Did we!" laughed Joe and proceeded to tell of the many drawings and inscriptions. "You boys should have taken the motion-picture cameras with you," Mr. Lewis said. "They would have furnished proof to the outside world." "Perhaps we can yet," said Joe. "No, you can't," protested Mr. Holton. "We must not waste any time here, if we are to find the hidden riches. Right now," he added, "you three had better turn in and take your afternoon rest. That sun is terrible!" Dr. Kirshner and the youths did as suggested, glad to rest their tired limbs. But they were up promptly at three, packing the tent and provisions on the dromedaries.