All along the way Bob and Joe took motion pictures of the many strange marvels of nature. They photographed the tall peaks, the deep gorges, and the narrow valleys. They turned the cameras on the many ancient inscriptions that were of so much interest to the archæologist. "We've run off a good many hundred feet of film," remarked Joe, after the cameras had "purred" for an unusually long period as the youths filmed a deep canyon through which they passed. "In fact, if they had given us twice as much film we could easily have used it all." Bob nodded. "And there's less danger of losing it in a hot, dry region like this," he said. "Isn't at all to be compared with the damp, tropical climate of Brazil." Slowly the country became more rough, until that evening, after the meal, the adventurers found it very difficult to pick out a way between the many large rocks. On one occasion they passed through a dark tunnel-like passage beneath high, overhanging peaks, and had literally to feel their way through. There was not the faintest ray of moonlight to lighten the deep passageway. When they finally came to the other end they gave sighs of relief that at last the moon could again be seen. But now something else hindered progress. A peculiar desert plant, with sharp, poisonous needles and a rather disagreeable odor, had been known previously to the explorers. Now, as they emerged from the dark recesses of the tunnel, they came upon large clusters of these plants. Everywhere the dromedaries went, it seemed, they were forced to tramp through these formidable shrubs. "It's a wonder they can stand it," said Joe, shaking his head. "Their feet are already swollen almost double." "Be tragedy if the animals would have to stop," the other youth said. "It would mean our end, I guess." "They'll make it all right," put in Mr. Lewis, who was riding at the rear with the boys. "But, needless to say, we couldn't walk on them." Suddenly, as they came to the edge of a dry river bed, Dr. Kirshner halted his camel and looked about on the ground. The others waited a moment to see what he had discovered. "An ancient tool of flint," he announced, commanding his camel to kneel. The archæologist picked up the instrument and examined it carefully with the aid of a flashlight. It was about a foot long and shaped to resemble a crescent. The inner edge was as sharp as a razor. "Doubtless some kind of a knife," Dr. Kirshner said, as the others crowded around him. "Shaped out of flint by people of the Neolithic age. Perhaps it was used to harvest crops." "Crops? In the desert?" Bob was amazed. The scholar nodded. "It is firmly believed that primitive people lived in what is now the desert rather than along river valleys of more modern prominence. Of course the Sahara was at that time much less arid, or dry, than it is today." Dr. Kirshner searched about for other relics, but found none.