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

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10

FORMATION OF ROCKS IN SOUTH WALES

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.^[1]

Fig. 4.

Fig. 5.

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

↑ The Rhone, at its entrance into the lake of Geneva, affords a good example of the manner in which the turbid waters of a river fall into a lake. Being specifically heavier with their usual load of detrital matter borne down by the glacier waters than those of the lake, volume after volume is seen to descend into the depths, and the surface remains clear while mud forms beneath, the turbid heavier waters forcing their way for a long distance under the still clear waters above, as is shown by soundings. In fact, the two waters resemble any two fluids of unequal density thrown into a basin together; and until the river water becomes lighter by the deposit of the matter held in mechanical suspension, it will tend to find its way over the bottom.

[1] The Rhone, at its entrance into the lake of Geneva, affords a good example of the manner in which the turbid waters of a river fall into a lake. Being specifically heavier with their usual load of detrital matter borne down by the glacier waters than those of the lake, volume after volume is seen to descend into the depths, and the surface remains clear while mud forms beneath, the turbid heavier waters forcing their way for a long distance under the still clear waters above, as is shown by soundings. In fact, the two waters resemble any two fluids of unequal density thrown into a basin together; and until the river water becomes lighter by the deposit of the matter held in mechanical suspension, it will tend to find its way over the bottom.

[1]