“This must have been the bed of the sea for a long time,” said the lad to Antoine, “this limestone deposit is so thick.” “Yes, yes,” his friend answered. “The Mediterranean came down south of this. Of course, the shore has changed many times.” “What has built up all the north coast of Africa,” queried Perry, “just the rising of the land?” “Partly; but a great deal of it was caused by the discharge of the mud of rivers and streams, making new land, in the same way that the Mississippi is making new land in the Gulf of Mexico. But you see, Perry, as fast as the new land was made, the continent sank, so that the filling and sinking went ahead at about the same rate. Sometimes the sea won out, and we find a layer that is marine; sometimes the land won out, and the layer is a river or fresh-water deposit. When we start to dig out fossils, you’ll probably find land and sea creatures close together. That’s because when it was a shallow sea, the marine creatures died and their bones sank to the bottom, and when it was marsh, the land animals died in the swamp and the bones became covered with mud. Then the sea took the upper hand again, as the continent sank.” “It’s beginning to look as though we were getting into the time when the land was ahead,” remarked Perry, examining the soil. “Yes, yes,” said Antoine, “you are right. And see!” He pointed to the face of the escarpment, up which they were toilsomely climbing. “Fossil trees!” cried Perry. “Yes, yes. All petrified. All these trees floated down that river millions of years ago. There were floods. There will be animals here, too, that were carried down by the floods.” He pointed to one of the many bones that could be seen. “The crocodiles were here,” he said, “so the bones are likely to be all separated, the dead animals having been pulled apart when they were eaten. It will not be easy, Perry. There must have been a sand-bar at the mouth of that old river somewhere about here, and when the animals came drifting down with the river gravel and sand, they were stopped and piled up at the sand-bar.” “Hello—” interrupted the boy, “what’s happening? It isn’t sunset yet!” The whole caravan had halted, as if at the time of Mohammedan prayer, and the men and boys fell on their knees. But, this time, the camels and donkeys crowded in and the lad saw that a few small rain-pools had been discovered. This unexpected supply of water cheered everybody, and it was only a little more than two hours later that the tents of the advance party came into sight. This party, carrying supplies and the heavy tools, had gone from Cairo to Tamia by train and hence had arrived three days earlier. Late that evening the tents were reached and the permanent camp pitched. It was on the widest of these ledges or tiers of rock, a ledge varying from one to two miles wide and stretching in an almost even line sixty to seventy miles long. The level of the Libyan desert was six hundred feet higher still, a stiff climb.