Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/20

Rh ward especially in floods, when the rivers and brooks are swollen and a greater volume and weight of water press onward. Coming into the still waters of lakes, the whole is checked, and a deposit of the mechanically suspended matter is effected. This matter is usually such as to form mud or clay,—very small sandy grains, when the rivers have become particularly rapid and full, sometimes being sufficiently added to produce silt.

The movement of sands and gravels onwards by means of running waters is usually by forcing these substances along the bottom of the brooks and rivers,—as it were, sweeping them forwards. At times the same substances may be caught up, from considerable movements in the water, in mechanical suspension; at others, from diminished motion of the water, be merely swept on; and between the one action, according to conditions, and the other, there may be frequent changes; but as a whole the great mass of sand and gravel appears to be shoved onwards in rivers by the friction of the superincumbent water.

In great lakes ample opportunities are afforded for seeing the varied manner in which a cavity in the course of the general drainage of the land is being filled up; in one place, the shore being steep, sands and gravels are forced into them, and accumulated at high angles, as in Fig. 1; in another, sands and clay are intermingled, as in Fig. 2; in more favourable localities, nearly horizontal beds of clay are formed, as also of limestone from waters in which sufficient calcareous matter is held in solution, to be deposited when the causes of this solution have ceased.

With the exception of the evaporation from the waters when in the lakes, those flowing into them pass out free from mineral substances in mechanical suspension; though the purity of such out-flowing waters, chemically considered, never equals that of rain water; and thus some of the mineral matter obtained in solution from the rocks traversed by the water, even in cases where deposits of calcareous or other matter have been chemically formed in the lakes, escapes into the sea.

Arrived at the sea, the detrital matter is materially influenced in its distribution, according as the coast to which it is carried is either washed