"Get out your goggles!" he directed them. "We seem to be riding into a fog of black dust. Tishmak thinks we may have to stop for the day." CHAPTER XV As Guests of Heathens "What!" cried Bob excitedly. "Stop for the day! We--we can't. Our time is worth too much!" The naturalist laughed unwillingly. "This time it's a question of what we have to do," he said dryly. "If the blackness increases, it will be suicide to plunge through it with so many rocks and walls of stone near." The boys and Mr. Holton rode on up to the front of the caravan, where Tishmak and Fekmah were visibly moved with fear. The darkness was becoming more intense with every minute. Soon the explorers could not see one another. "It might be well to get off our dromedaries," suggested Mr. Lewis. "If this is to continue long, it won't pay to remain mounted." "This is likely to mean a loss of several hours," said Fekmah gravely. "Then again, the fog of dust might pass away in but few minutes." "Worst thing about it is the difficulty to breathe," pointed out Dr. Kirshner. "Tragedy has been known to come upon caravans in this manner." It was indeed stifling, but the adventurers managed to get air through the dust. They feared every minute that time would bring about an impossibility to breathe. Much to their relief, that dread moment did not come, and after a half-hour the blackness gradually thinned out until it again became light. "Thank goodness!" breathed Mr. Lewis. "Now let's get on our way again." Still more the country was changing. The rocks became more jagged, taking on many shapes and forms. Small trees and plants became more abundant, causing the region to resemble certain areas of the American desert. And along with this plant life came numerous traces of wild animals. "Wish we would run across a fox," said Joe. "I'm anxious to shoot anything." "Probably will," returned Bob. "I guess there are not only foxes but jackals and other small game. There should be much more here than in the sandy section." The party stopped at eleven o'clock for the meal and midday rest. This time the dromedaries alone were to be the guards, for they had already proved their efficiency in detecting unusual noises. At two they were again on their way, climbing a gentle slope widespread with sharp rocks. "Seems strange to get away from the sand dunes," remarked Bob. "It hardly is like a desert now." As they passed up a rocky hill, Fekmah called a halt. "We have come to the Tudemait Plateau," he announced, getting out the map that he had made from memory. "That sharp peak in the distance is one of the landmarks. Now we pass through this plateau for rest of today and part of tomorrow; then come to another sandy stretch." "Then what?" inquired Mr. Holton. "Get to Ahaggar Mountains," the Arab replied. "We have to follow compass more close from now on, because must see peak called Illiman. Then we know we near treasure."