

448 Proceedings oj the Royal Irish Academy. 



accumxilations, however, are greatly modified during floods in the 

 rivers, as a large freshet may effect considerable denudation during 

 the time the tide is out. Considerable denudation can also be effected 

 by artificial means ; for, by judicious arrangement, as in the Boyne, 

 counties Louth and Meath, the tidal waters can be changed into cui'- 

 rents that act like freshets in rivers. 



Scott Russell has shown that, at the centre, a " wave of transla- 

 tion" is stronger and swifter than at its margins; somewhat in the 

 same way, the driftage of the in-coming tidal wave off-shore is usually 

 much stronger than it is in-shore. This is a fact well known to the fisher- 

 men, who often dredge and fish worked-out ground, rather than fish fresh 

 ground further out, on account of the additional labour that would be 

 incurred in the latter place, consequent upon the augmented velocity of 

 the tides. In-shore, if there is a wide and long shelving beach, the 

 driftage effected is spread over a large expanse, and the results are not 

 very conspicuous or easy to study. This, however, is not the case on 

 quickly-shelving beaches, where the driftage solely due to the tidal 

 - wave is conspicuous, as when there is not a breath of wind blowing ; 

 if the beach is composed of fine sand or gravel ; each wave, according 

 to its intensity, carries up numerous particles in more or less oblique 

 lines. The major portion of the particles go up and come down, as 

 represented by the curved arrows (y and g', fig. 5) ; some, however, re- 



main behind, and eventually reach the top of the beach, in a track 

 somewhat similar to that marked by the curved arrows (/, /' and/"). 

 In a mixed beach, most of the fragments go with the arrows 

 {g and g') ; but many of the larger fragments ascend the beach in the 

 irregular course indicated by the arrows (/, /' and/"), especially dur- 

 ing spring tides ; . these lodge on the top, and form a more or less 

 marked gravelly or shingly margin. It is, therefore, not unusual to 

 find a sloping beach constituted as follows : — Above, at the margin of 

 high-water of spring tides, a more or less well-marked terrace or accu- 

 mulation of coarse gravel or shingle [h, fig. 5) ; at the high-water of 



* « in this figure represents the shore-line, to the left-hand the three perpendi- 

 cular black lines represent artificial groins. 



