KiNAHAN — Tidal Currents and Wind-ivaves. 449 



neap tides, a second (c) ; this, however, is small and temporary, as it 

 is obliterated and removed duiing springs. Between high and low- 

 water of neap tides (c and d ), there is a shelving beach of gravel 

 and sand, and immediately below d is an accnmiLlation of shingle 

 or coarse gravel, forming the base of the steeply- shelving beach; 

 while between the latter and the line of lowest spring tide is a flattish 

 space, often composed of fine, immoveable sand, the outside or lower 

 margin of the regular-moving beach being at the sliingie accumulation, 

 below the line d. On the slope between c and fZ there are always more 

 or less large isolated fragments scattered about the surface ; the shingle 

 at h, except under peculiar circumstances, will ordinarily be much less 

 in quantity than at d ; the "latter moves, more or less, every tide, while 

 the upper accumulation (h) only is moved during springs. After 

 storms, however, patches of gravel and shingle will be scattered over 

 the slope between the line c and d. 



This travelling of beaches accounts for the accumulation of shingle 

 on the up-stream side (relatively to the direction of drift movement) of 

 an artificial groin, which stops the travelling of the materials of a beach, 

 as the larger fragments ascending a beach are forced to travel along 

 the coui'ses indicated by the arrows {Ji and i, fig. 5), and are prevented 

 fi'om descending by the groins, while the smaller particles ' are 

 sucked out by the backwash, some accumulates in the space above 

 the arrow (A). Or, if the groins are at considerable distances apart, the 

 materials are assorted by the offshoots {h and;?;), from the main current 

 (j), the larger particles going with the latter. Many natural groins 

 act somewhat similarly, but as some run OTit into deep water for 

 greater or less distances beyond the margin of the beach, their action is 

 not alike in all cases, as will appear from the following. Usually as a 

 beach while travelling meets with a natural groin (as the headland 

 a, fig. 6), the sand, &c., is sucked out fo sea in the direction of the 

 arrow {g), to be diiven into the next bay obliquely (Ji). If, however, 

 the bay {e) is narrow and regularly formed, the wash will be directly 

 in and out {i andy), and in such bays the tidal action seems to tend to 

 accumulate larger beaches than in more open bays. If there is a suc- 

 cession of bays {d, e,f, &c.), with strands, and the headlands between 

 them (ff, h, c, &c.) are formed of materials not easily denuded, there 

 will be no source to supply shingle, consequently the materials in the 

 beaches {k, I, m, &c.) will decrease in size from wear, till eventually the 

 beach will be solely composed of fine sand, without shingle or gravel 

 margins, either above or below {b and d, fig. 5). This refers to the drift- 

 age along the margin of the coast ; there might, however, be deep sea 

 driftage of coarser material, that would be carried obliquely on to the 

 coast, that may modify the above results ; as shingle and gravel often 

 are carried direct from one headland to another, although very far 

 apart. 



So far the driftage considered has been that solely due to the tidal 

 currents ; their action, however, can be modified or augmented by wind- 

 waves. Wind- waves, as shown by Scott Eussell, are usually waves of the 



