416 A. R. Hunt — Nomenclature of Ri2jple-mark. 



Devonian rocks without a trace of shallow-water conditions. Asso- 

 ciated with these corrugated rocks there is a band of some inches 

 of badly preserved shells suggestive of some great destruction of 

 the submarine fauna ; what Gwyn Jeffreys would have described 

 in modern seas as a charnel-house of shells. The currents were 

 clearly sharp, but transitory, as the grit and slate beds are not 

 confused, and the thickness of the shell band very regular. The 

 corrugations are symmetrical, so must be wave-formed and not 

 continuous current-formed, that is, if they are ripple-mark at all. 

 Now in considering such a case as this we have to realise the 

 presence of waves heavy enough to disturb depths at which fine 

 silt and mud can accumulate. This depth, disturbed only on rare 

 occasions, will depend on the height (crest to trough) of the waves. 

 The amplitude of the reciprocating currents over the bottom will 

 depend on the height (crest to trough) of the waves and the depth 

 of the water, while the number per minute of the double currents, 

 or their frequency, will depend on the period of the waves. Now 

 the technical terms required for this description are height (crest 

 to trough), length, amplitude, period, and frequency. The terms 

 height and length will apply to ripple-marks equally well ; but with 

 amplitude, period, and frequency ripple-marks have nothing to do. 

 If we use amplitude for the height of a ripple-mark we use a stereo- 

 typed wave-term in a different sense ; while, if we use the term 

 wave-length for the ripples, our thoughts ai'e at once directed to the 

 true waves which formed them, waves which really possessed 

 wave-length, which the ripple-marks only possess by courtesy. 



My own work in ripple-mark, which was undertaken solely to 

 establish the doctrine of alternate wave-currents, received its full 

 fruition when Sir Archibald Geikie accepted the doctrine of the 

 " oscillation of the medium " in his textbook of 1893. That fact 

 accepted, all the rest, the interesting consequences, must follow in 

 time. But they will follow sooner if we can avoid confusion of 

 ideas being perpetuated by ambiguous and even conflicting nomen- 

 clature. Were this a paper on ripple-mark itself it would be easy to 

 run through the great textbooks and manuals and indicate where 

 the different authors have followed the wrong trail. I Vi^ill, how- 

 ever, quote one very useful and popular dictionary of scientific 

 terms. In Webster's Dictionai'y, ed. 1876, we read, "Ripple-mark. 

 (Geol.) A mark on the surface of a rock resembling that made by 

 receding waves on a sea beach." Now waves on a falling tide have 

 often effaced ripple-marks on the flats of a sea beach, but have never 

 created them. The efiicacy, or even the existence, of a receding 

 wave is as imaginary as that of the efficacy of the advancing or 

 receding tide, moving towards or from the shore at the rate of a few 

 hundred feet or less in some six hours. The wave has ceased to 

 exist before any water recedes from off the beach. 



So far as I am aware, no paper on ripple-mark has appeared in 

 any geological publication since Dr. Sorby's " Structures produced 

 by the Currents, etc.," in the Geologist in 1859. The literature 

 is scattered far and wide. Sir Archibald Geikie has given the 



