TRANSACTIONS OF SECTION C. 677 



tjuestion he settled, no progress can he made in assigning to waves their proper 

 position among agents of denudation and deposition. 



The author said that much of the prevailing uncertainty arose from the fact 

 that waves of oscillation have not heen studied experimentally, and that Mr. 

 Scott Russell's waves of translation, created by admitting a fresh volume of water 

 into a tank, are not analogous to sea waves in any part of the passage of the latter 

 from deep water to the shore. One proof of this is that the ordinary sea wave of 

 oscillation is always preceded by a depression, and plunges seaward of the margin 

 of repose of the water ; whereas one characteristic of J\ii\ Paissell's wave of trans- 

 lation is that its surface is wholly raised above the level of repose of the fluid. 



The author showed by diagrams that experimental oscillating waves plunged 

 further and further from the margin of repose as the incline of the beach was 

 reduced, and that they showed no tendency to turn into waves of translation. 



The subject was then considered from four points of view, viz., observation, 

 direct and indirect ; experiment ; and theory. 



(1) Direct Observation. — As one of innumerable instances of bottom disturb- 

 ance, the author referred to slate shingle di-edged 2,600 yards east of Hope's Nose 

 in Torbay (in 14 fathoms), together with a specimen of the sluggish and helpless 

 mollusc Pleurobranchus 7nembranaceus. The shingle was derived from the northern 

 shores of Torbay, and proved that in heavy weather the shingle occasionally tra- 

 velled into deep water. The mollusc proved that in ordinary weather the tidal 

 currents exerted no appreciable disturbing action. 



(2) Indirect Observation, — The forms of marine faima exposed to wave currents 

 are specially adapted to withstand such action. 



(3) Experimental. — Artificial oscillating waves made to roll over glass plates 

 disturb flocculent matter on the plates at a depth approaching half the wave- 

 length. 



(4) Theoretical. — According to theory the disturbance in deep water is very 

 slight at half the wave-length, but rapidly increases on approaching the curface. The 

 evidence of rolling action at forty fathoms on the bottom of the English Channel, 

 based on the condition of a soda-water bottle and its contents (exhibited to Section 

 C at Southampton, 'Eep. B. A.,' 1883, p. 535), was afterwards fully confirmed on 

 mathematical grounds by Professor G. G. Stokes, Pres.R.S. (' Journal Linnean 

 Soc. Zoology,' vol. xviii. p. 263). 



After describing the complexity of the currents set up in front of plunging 

 waves, the author pointed out that behind the plunging point the waves stir up the 

 sand by symmetrical oscillating currents, which keep it in motion and place it at the 

 disposal of any passing continuous current, whether derived from tide, wind, or 

 even earthquake. Of these currents perhaps those raised by wind are of most 

 importance, as they are most intense when the waves themselves are highest. The 

 water propelled by the wind shorewards (independently of any wave motion) 

 escapes seaward as an under-current, or side-current along shore. Thus in wind 

 and wave combined we have a most eflicient excavating tool — a tool which not 

 only cuts well, but which clears itself well. 



The waves cut the land, the wind-raised currents remove the dibris, and 

 this frequently in tlie teeth of both wind and wave. 



If waves can disturb sand and shingle at a depth of forty fathoms, as the triple 

 cord of evidence relied on seems to prove, and if these waves are accompanied by 

 wind-formed currents, as they often are, then waves supplemented by wind- 

 currents are agenta of denudation and deposition, which no geologist can atford to 

 neglect, except at the risk of seriously misinterpreting the records of the sedimentary 

 rocks. 



10. Third Beport on the Bate of Erosion of the Sea Coasts of England and 

 Wales. — See Appendix, p. 847. 



