The only question for us to consider is, how this was brought about. It could have been brought about only in one way, and that was by the river deepening its channel. As far down as Silsiléh it had been flowing at a higher level. Here there must have been a cataract, or an actual cascade. Whatever the form of the obstruction, the stream carried it away. And so, again and again, working backwards, it ate out for itself a deeper channel all the way up to Semnéh. This is just how the Niagara river is dealing with its channel. It has undertaken the big job of deepening it, from Lake Ontario to Lake Erie, down to the level of Ontario. The stone it has to work in is very hard and compact. It has now done about half the work, and every one sees that it will eventually complete it. All that is required is time. The River Colorado, we are told, runs for six hundred miles of its course in a canon, a mile in perpendicular depth, all cut through rock, and some of it granitic. This is what the Nile did in the historic period for at least two hundred miles of its course. It planed down this part of its channel to a lower level, to what may be called the level of Egypt. Why should it not have done precisely the same work in the prehistoric period for, in round numbers, the four hundred miles from Silsiléh to Cairo, that is to say, for the whole valley of Egypt? That is just what I believe it did. Of course, there were aboriginal facilities which decided it upon taking that course. There may also have been greater depressions in some places than in others. There was harder work here, and lighter work there. The planing was carried on rapidly in one district, and slowly in another. But I believe that, after making whatever deductions may be thought proper for aboriginal depressions, it is safe to conclude that the valley of Egypt was, in the main, cut out by the Nile. It did not begin to obtain its abrading power after the reign of Amenemha III. There may have been a cataract once at Cairo. When this was carried away, another must have been developed somewhere above its site, and so on backwards all the way to Silsiléh, where we are sure that there was once something of the kind. In a still remoter past the river may not have come as far north as Cairo, but may have passed through the Faioum, or by the Natron Lakes, into the desert. This is a question which, to some degree, admits of investigation. The river would not always be bearing on the same side of the valley. A little change in any part of the channel, and which might result from any one of a variety of causes, would deflect its course. It is so with all rivers. These causes are always everywhere at work. The river would thus be always shifting from one side of the valley to the other; and, impinging in turn on the opposite bounding hills, would always be widening the valley.