I.] WEATHERING 13 



settle, the finer particles are only deposited when the 

 velocity of the stream has been almost entirely checked. 

 The gravel and sand are deposited in and about the 

 stream course itself, the finer material falls on the meadows 

 in flood time, so that their level is gradually raised from 

 year to year. Wherever the meadows get water-logged 

 the surface vegetation will begin to accumulate as 

 peat ; the stream also wanders about from side to 

 side of the valley, hence borings through any exten- 

 sive deposit of alluvium will disclose alternating beds 

 of gravel, sand, brick earth, and peat, of variable 

 extent and thickness. The great alluvial flats or 

 marshes at the mouths of many of our rivers are 

 formed in this manner ; the deposit takes place in the 

 sea or in the estuary, until the tides and currents work 

 the material up to high-water mark, after which only 

 fresh-water beds are laid down. 



Although most of the materials of which rocks are 

 composed are in the ordinary sense insoluble in water, 

 few of them, except the pure sand grains, can resist the 

 attack of water charged with carbonic acid. The rain 

 water when it reaches the ground has little carbonic acid 

 in solution, but the gases in the soil contain a consider- 

 able quantity derived from the decay of vegetable matter 

 in the surface layer, and the water in contact with these 

 gases will dissolve a proportionate amount. The pro- 

 portion of carbonic acid in the soil gases varies very 

 much both with the permeability of the soil and the 

 proportion of humus, but at a depth of 1-5 metres 

 Wollny found it vary from 3-84 per cent, to 146 per 

 cent, at various periods of the year. At greater depths 

 the amount is still higher, so that the percolating water 

 becomes a weak solution of carbonic acid, and attains a 

 considerable solvent power. Not only are the alkaline 

 silicates attacked by the weak acid thus formed, but as 



