48 Notices respecting New Books. 



drained if the Falls of Niagara should retreat to it ; and it is shown 

 that, far from producing a deluge or debacle, as has been often sup- 

 posed, the result would be a drainage, which, though more rapid at 

 certain intervals, would still in the main be gradual, finally leaving 

 a river flowing through the bed of the discharged lake. 



Respecting the embouchures of rivers, it is observed, that " if the 

 mouths of rivers be tidal, the river detritus is committed to the charge 

 of the estuary tides, and is dealt with according to the laws by which 

 these are governed. If they be tideless, the whole mass of transported 

 matter will be propelled without check into the seas at the embou- 

 chures. Between the extremes of great resistance and non-resistance 

 the variations are so great, and depend so much on local circumstances, 

 as to be of exceedingly difficult classification. The principal variations 

 are produced by the difference in the volume of the discharging rivers, 

 their velocities, and the quantity and quality of the substances they 

 may transport. As a general fact, however, it may be stated that 

 rivers tend to form deltas in tideless, or nearly tideless, seas ; or 

 where they can overcome the resistance of tides, currents, and the 

 destructive action of the breakers ; thus increasing the land by their 

 deposit, and splitting into several channels, the superficial increase 

 being in proportion to the depth of water into which the rivers dis- 

 charge themselves." 



"In calculations of the advance of deltas, care has not always been 

 taken to show the general depth of water into which they may have 

 been protruded ; so that a less quantity of transported detritus might 

 expose a larger surface when thrown on a shallow bottom, than a larger 

 quantity in deeper water." 



The author observes, under the head of Beaches, that " the action 

 of the waves round coasts tends to disturb the bottom at certain 

 depths, and to move the shells, sands, and other substances of which 

 this bottom is composed, towards the land. The exact depth to which 

 the moving action of waves extends, seems never to have been very 

 accurately estimated : indeed, when we consider that the power 

 of the wave is continually varying, such an estimate becomes exceed- 

 ingly difficult. Ninety .^eet, or fifteen fathoms, has been sometimes 

 considered as the limit, in depth, to which this disturbing power ex- 

 tends; but this requires confirmation. Around coasts and on shores 

 which do not much exceed ten or twelve fathoms, the action of the 

 waves is very apparent in the discoloration of the water during heavy 

 gales. This turbid character of the sea is due to the moving power 

 of the waves on the bottom, and becomes more marked as the water 

 becomes more shallow, either in approaching the land or over shoals. 

 The transporting j)ower of the waves will therefore be in proportion 

 to the depth of water beneath them, the transport being greatest in 

 the shallowest places. The waves will tend to throw substances on 

 coasts, because the off-shore wind produces smaller waves than the wind 

 blowing upon the land. On shoals distant from the land the effect 

 will be somewhat different, and the piling or propelling power will be 

 greatest on the side of the prevalent or more violent winds. Shoals 

 will be also liable to shift, as the turbid waters on the crown of a 



shoal 



