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ries. If, on the other hand, currents be continuous and constant, the zones of sand, clay and silt deposits will occur each beyond the former. But this is a question of distribution as well as of sorting of sediments.

(c) Moderate volumes of sediments.—Sediments are also more or less completely sorted by waves or currents according to the relation between the volume of sediment and the force of the sorters. When waves breaking upon a coast have only the product of wave erosion to handle they sort most completely; the material is washed again and again until no trace of clay remains mingled with the sand grains; and the under-tow, burdened only with the clay washed out by the waves and the fine products of abrasion, carries them all away. But where a river pours out a large volume of sediment, and waves or currents are consequently overloaded, both sorting and transportation fail to a greater or less degree. Deposition takes place too rapidly for the separation of fine from coarse and the deposit is of mixed character. The effect of waves is then seen in ripple-marked and ill-assorted beds of tide flats.

The conditions under which sediments are more or less widely distributed, depend upon movement of the waters and the nature of the sediment; those favorable to dsitribution are:

Favorable conditions:

The reverse of these conditions favor deposition, and will be discussed in that connection.

(a) Efficient, oblique wave action.—Distribution of shore drift is fully discussed by Gilbert, and has already been referred to in stating the effect of sorting by waves of the Atlantic on