THE EGYPTIAN SUDAN he had mounted it he pulled the boat up to it ; we then held on to the rock whilst he swam on to another and pulled us up to it as before.

Between these rocks the current was rushing at the rate of five or six knots an hour, and in some places we had to leave the main stream and creep along the side channels. We beguiled the tediousness of the journey by examining the huge blackened sandstone natural barriers in the river as we passed quite close to them, or touched them, and we saw at work the forces which had enabled the Nile to break its way through them. It seems that small pieces of hard stone and pebbles are brought down by the river, and deposited in the inequalities of the surface of the top of the sandstone. As the water flows over them it imparts motion to them, and by degrees they bore in the sandstone holes which become deeper and deeper, and we saw some holes several feet long, the sides of which had been rubbed through by the stones inside them. At low Nile the sandstone is exposed to the heat of the sun by day and to the cold by night, and the sides of the holes split open and they run into one another, and pieces split off their sides until at length gaps are made in the barriers. During the inundation the torrent pours between the rocks in a nine knot current, the boring of new holes begins, and the sandstone is scoured away. These facts were pointed out to me by Mr. John Ball, of the Egyptian Geological Survey Department, who was, I believe, the first to ascertain them.

In the course of the afternoon we approached a " gate " in the Cataract through which the river was rushing with mighty force, and the captain insisted that, with two or three exceptions, every one should leave the boat whilst he tried to work her through. We therefore got out on the rocks and watched the result of his labours with some anxiety, for though he was confident of success,, we had our doubts about the matter; our beds, baggage, and tools we left with him, for the boat was some distance from a suitable landing-place. We had a coil of 60 yards of thick rope with us, and having tied one end of this to the main cross-beam of the boat, the captain told every man available to haul on it, whilst he pushed out a little into the current, and gradually poled the boat along the sides of the rocks towards the " gate." When 474