Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/23

10 animal life suited to different conditions, the sands and gravels often forming extensive beds, with diagonal partings. When considering the power of transporting the finer sedimentary matter in mechanical suspension, we should recollect that in seas particles can be kept longer in mechanical suspension by a given motion of water than in fresh water, from the difference of the relative specific gravities of the two fluids; and, consequently, with the same amount of onward movement, that detritus can be carried greater distances by sea-waters, all other things being equal.

With slight modification, detritus borne into lakes and tideless seas may be considered as accumulated in the same manner, though there may be differences arising from chemical causes; changes being effected in the seas, among the fine sedimentary matter, from the substances in solution, to which similar matter in lakes may not be exposed.

The deposits of sedimentary matter in tidal seas would so far differ from those in lakes and tideless seas that they would be modified by the action of tidal currents, by the greater amount of coast brought within the destructive action of the breakers, during the rise and fall of tides, and by the ponding back of rivers to various distances from their embouchures during flood tides, causing deposits of detritus in estuaries, and the discharge of the ebb-tide waters in particular directions.

The tidal wave becomes a power sweeping forward sedimentary matter along sea bottoms and estuaries, in proportion as the depths are shallow, and as there arises much friction on the bottom. In arms of