It is so very important, for those who wish to see things "as clear as mud," to understand this prime principle of the formation of mud-lands, that I shall make no apology for insisting on it further in some little detail; for when one comes to look the matter plainly in the face, one can see in a minute that almost all the big things in human history have been entirely dependent upon the mud of the great rivers. Thebes and Memphis, Rameses and Amenhotep, based their civilisation absolutely upon the mud of Nile. The bricks of Babylon were moulded of Euphrates mud; the greatness of Nineveh reposed on the silt of the Tigris. Upper India is the Indus; Agra and Delhi are Ganges and Jumna mud; China is the Hoang Ho and the Yang-tse-Kiang; Burmah is the paddy field of the Irrawaddy delta. And so many great plains in either hemisphere consist really of nothing else but mud-banks of almost incredible extent, filling up prehistoric Baltics and Mediterraneans, that a glance at the probable course of future evolution in this respect may help us to understand and to realize more fully the gigantic scale of some past accumulations. As a preliminary canter I shall trot out first the valley of the Po, the existing mud flat best known by personal experience to the feet and eyes of the tweed-clad English tourist. Everybody who has looked down upon the wide Lombard plain from the pinnacled roof of Milan Cathedral, or who has passed by rail through that monotonous level of poplars and vines between Verona and Venice, knows well what a mud flat due to inundation and gradual silting up of a valley looks like. What I want to do now is to inquire into its origin, and to follow up in fancy the same process, still in action, till it has filled the Adriatic from end to end with one great cultivable lowland.