KiNAHAN — Tidal Curreids and W ind-waves. 457 



mile Point,* on the north of Wicklow, and flows southward to Bride 

 Head, off which there is a considerable "race." This " counter- tide " 

 drifts the beach southward, and thereby has formed a shingle and gra- 

 vel ridge, which has pushed the Yartry southward, until its farther 

 progress in that direction was stopped by the hard rocks that form the 

 "Wicklow Head promontory, the pent up waters forming a lagoon and 

 marshes. Here, at the south end of the ridge, the driftage seems to 

 be carried to sea; for, as in other places where a rock-bound coast stops 

 the further growth of a ridge, the embouchure out of the lagoon seems 

 to have a permanent bar : but, what here appears remarkable, this bar 

 has not changed during late years, although half the waters that used 

 to come down the Yartry are now diverted to supply Dublin. 



These notes record what has been going on during late years, but 

 it has to be remembered that, during comparatively recent years, large 

 bays existed, that are now filled up with ^olian sand.f "Why the 

 sand banks accumulated in these bays seems to be due to a slight rise 

 in the height of the land, as, when the sand has been removed by wind, 

 ancient sea beaches are found under them. If, therefore, their accu- 

 mulation was due to a rise in the land level, it appears probable that in 

 those places where the sand banks are now being denuded, there must 

 be a slight fall going on in the level of the land. The difference, how- 

 ever, in the levels is so slight that changes in the height of the Rise 

 and Fall of the tide (Full and Change), possibly, might account for the 

 denudation, without the level of the land changing. 



Eesults. 



The information gathered on this portion of the coast of Ireland 

 goes to prove the following : — 



First. — The driftage due to the incoming tidal current is always, 

 during its progress, going on in deep water, and more or less in shal- 

 low water. 



Second. — The driftage due to wind-waves only occurs during gales, 

 and even then is only due to the waves that break on the shores. 



Third. — To prevent the tidal driftage (arrow A Fig. 7), groins, or 

 piers, should be erected ; and if the pier (b) is to form a harbour (/), 

 transverse groins {c c^) should run out from it, to stop the back wash 

 generated by the pier, for otherwise this back wash would carry the 

 driftage seaward, in the direction of the arrow {d), to be sucked round 

 the pier into the harbour (/). 



Fourth. — As the wind- wave driftage occurs during gales, and then 

 only on the shore line, it might be prevented from filling up a harbour, 



* Admiralty Chart, Sheet xvi. 



t About a mile S.S.W. of Kilmichael Point, the writer was shown a rock cliff, 

 recently exposed by the denudation of the ^olian drift, that, prior to the accumu- 

 lation of the latter, seems to have been quan-ied b}' man. 



R.I. A. PROC, SER. II., VOL. II., SCIENCE. 3 B 



