THE ORIGIN OF SOILS 191 



land surface upon which we now live either emerged from the sea 

 or was left as naked rock on the melting of its last covering of 

 glacial ice. As a matter of fact, the soil covering we see on the 

 side of the hill is only a fraction of that which has been formed 

 from the rock, because the soil is always creeping down the hill- 

 side and being washed away by the river below. In time of 

 heavy rain this is obvious enough, every little drainage gulley 

 in the fields, each spring and streamlet well above the main 

 river, are charged with turbid water ; the river itself is not only 

 heavily laden with sediment washed from the land, but is pushing 

 along its bed sand and gravel and even stones of considerable size. 

 The broad truth that rivers have carved out their own valleys, 

 and that the material they remove is not rubbed off the solid 

 rock, but is the soil into which the rock has first of all passed 

 by the action of the weather, itself indicates that the soil-making 

 process must have been a far greater one than can be measured 

 by the amount of soil actually existing to-day. 



Having thus seen that in times of heavy rain a great deal of 

 the soil which had been previously formed gets washed into the 

 rivers, it is instructive to collect some of this rapidly flowing 

 turbid water and let it settle down in a clear glass in order to 

 get an idea of the nature and amount of the sediment that is being 

 transported. In most cases, when the water is drawn from a fair 

 sized river in not too violent a state of flood, the material will 

 take some days to settle down, and will then form a sticky deposit, 

 which, however, is not wholly clay, because it feels a little gritty 

 when rubbed between the fingers. It consists, in fact, of the finest 

 particles of the soil ; the stones, the gravel, and even the coarser 

 sand have been left behind or are being pushed more slowly along 

 the river bed. It will be dark coloured, because it has been chiefly 

 washed off the surface layer of the soil containing vegetable matter, 

 and if it is dried and heated in an open dish the presence of this 

 vegetable matter will become evident by the charring smell. 



As to the fate of this solid matter suspended in the fast 

 running water, there can be but little doubt : much of it evidently 

 finds its way down into the sea, and is there deposited as the 

 mud flats, sand banks, and shoals which impede the mouth of 

 many of our rivers. If the tides and currents off the mouth of 



