For remember that, from a point above Berber to the sea, the dwindling Nile never receives a single tributary, a single drop of fresh water. For more than fifteen hundred miles the ever-lessening river rolls on between bare desert hills and spreads fertility over the deep valley in their midst--just as far as its own mud sheet can cover the barren rocky bottom, and no farther. For the most part the line of demarcation between the grey bare desert and the cultivable plain is as clear and as well-defined as the margin of sea and land: you can stand with one foot on the barren rock and one on the green soil of the tilled and irrigated mud-land. For the water rises up to a certain level, and to that level accordingly it distributes both mud and moisture: above it comes the arid rock, as destitute of life, as dead and bare and lonely as the centre of Sahara. In and out, in waving line, up to the base of the hills, cultivation and greenery follow, with absolute accuracy, the line of highest flood-level; beyond it the hot rock stretches dreary and desolate. Here and there islands of sandstone stand out above the green sea of doura or cotton; here and there a bay of fertility runs away up some lateral valley, following the course of the mud; but one inch above the inundation-mark vegetation and life stop short all at once with absolute abruptness. In Egypt, then, more than anywhere else, one sees with one's own eyes that mud and moisture are the very conditions of mundane fertility. Beyond Cairo, as one descends seaward, the mud begins to open out fan-wise and form a delta. The narrow mountain ranges no longer hem it in. It has room to expand and spread itself freely over the surrounding country, won by degrees from the Mediterranean. At the mouths the mud pours out into the sea and forms fresh deposits constantly on the bottom, which are gradually silting up still newer lands to seaward. Slow as is the progress of this land-forming action, there can be no doubt that the Nile has the intention of filling up by degrees the whole eastern Mediterranean, and that in process of time--say in no more than a few million years or so, a mere bagatelle to the geologist--with the aid of the Po and some other lesser streams, it will transform the entire basin of the inland sea into a level and cultivable plain, like Bengal or Mesopotamia, themselves (as we shall see) the final result of just such silting action.