Enough has now been said to show that stupendous quantities of mineral matter derived from the destruction of the land are sent down to the sea by natural agencies, and we may at once state that a very large proportion of this, which finds a resting-place in and about the mouths of the rivers and their backwaters, is material suitable for brickmaking at places where it is obtainable. Enormous quantities of muddy sediment, sand and gravel, however, never reach as far as the sea with great rivers. This material is arrested at sundry convenient spots, and, as a rule, forms excellent brick-earth. [Illustration: Fig. 1.--Formation of Brick-earth in a river valley.] See Fig. 1, which represents part of a river of slow current with three bends, A, B, C. The water is flowing in the direction indicated by the arrows; and it is part of the mechanics of such a river that in rounding a bend its velocity is greatest (and its eroding power also) at the outer portions of the curves approximately indicated by the arrow points. The water “wheels round” such portions of the curves, and “marks time” at the points _x_ _x_ _x_, and, indeed, its progress may be altogether arrested for a time at the latter places. Now the transporting power of a river is its velocity, and, naturally, the greater the velocity, the coarser will be the fragments or particles of rock carried along. It is interesting in this connection to quote the figures calculated by Mr. David Stephenson, giving the power of transport of different velocities of river currents:-- Ins. per Mile per second. hour. 3 = 0.170 will just begin to work on fine clay. 6 = 0.340 will lift fine sand. 8 = 0.4545 sand as coarse as linseed. 12 = 0.6819 will sweep along fine gravel. 24 = 1.3638 will roll along rounded pebbles 1 inch in diameter. 36 = 2.045 will sweep along slippery angular stones of the size of an egg.