1849.] AUSTEN ON THE VALLEY OF THE ENGLISH CHANNEL. 75 



find notices of " races " as occurring at the very spots at which we 

 now meet with them ; tending to show, that whereas the action of 

 breakers along the coast-line has, vidthin comparatively short times, 

 produced great changes, by the removal of thick masses of strata, 

 yet that no corresponding abrasion has been effected over these 

 ledges under depths of not more than 17 fathoms. 



The difference of velocity between the upper and lower strata of 

 water put in motion by the tidal stream, as in every current, is less 

 over the bed than at the surface ; but the surface velocity, even in 

 parts of the Channel where it is greatest, cannot be estimated at more 

 than from five to six miles an hour, so that its movement over the 

 ground or as a mechanical agent is very trifling. 



The two actions, first, that of wind-waves when they break in 

 shallow water ; and, secondly, that of the tidal stream, are alone en- 

 gaged in fashioning and arranging the materials of the sea-bed*. 

 The principal action of the wind-waves, as exercised on the land, is 

 confined to a zone extending from one range of the tide to the other. 

 In a zone below this, a certain amount of shifting, and consequent 

 abrasion of the materials, may take place ; the breadth of this zone 

 will depend on the nature of the coast, whether sheMng or other- 

 wise ; for with the deepening of the bed beyond the line of wave-un- 

 dulation this direct littoral action ceases. It would be difficult to lay 

 down the precise zone of depth along which the action of wind-waves 

 may be considered to cease ; but as in high waves of this order, at a 

 depth equal to one-third of a wave-length, the range of oscillation of 

 the particles is only one-thirtieth of that of the particles on the sur- 

 face, the depth to which water can be affected in any part of the 

 Channel cannot be very great. Where the action of the wind-waves 

 ceases, the permanent influence of the tide-stream begins ; and as this 

 extends to every depth and portion of the Channel, it has for its limits, 

 as an agent of accumulation, only the length of time during which 

 particles of matter may remain in suspension : in short, in every sea 

 the power of abrasion is confined to a marginal zone, and that of 

 dispersion, though with a wider range, extends only to the minuter 

 particles of matter. 



I am aware that a very different view of the movement of the water 

 of the Channel has recently been given, and that in support of it the 

 areas of discoloured water over certain banks at great depths, as well 



* It is only proper to state, that M. Emey, who has treated the subject of the 

 movement of the waters of the sea in great detail, so far as bars, sand- and shingle- 

 banks, and even erratic blocks are concerned, attributes them to the action of 

 what he terms his ground wave {fiot defond) ; that is, to the motion of the water 

 near the bottom towards the shore. (Mouvement des Ondes, p. 51.) M. Emey 

 adduces, as practical tests of his theory, first, that when bathing in the sea, at 

 a short distance from the shore, and with the body upright, we are lifted up by 

 the surface-undulation, whilst at the same time the horizontal passage of the 

 ground wave to the beach is felt by the legs. Again, if two pieces of cork, one 

 weighted so as just to sink to the "bottom, be thrown into the sea, the floating 

 piece will keep its distance from the shore, and only follow the surface-undulation, 

 whilst the weighted one will be rolled along the bed, and thrown up on the 

 beach. 



