18 



Dr. Tyndall. 



[Apr. 20, 



by the sinking waters of an inland lake or sea, may often be distinguished 

 from those that have been formed and preserved under water. The 

 former are usually imperfect through loss of their sharp ridges.* 

 Occasionally in addition to this they form natural channels for surface 

 drainage, and whilst their ridges are levelled, their furrows are 

 deepened. This fact seems to be referred to by Dr. Geikie, in the 

 third edition of his " Manual of Geology," where he points out, that 

 owing to the ridges of fossil ripplemarks being often broad and 

 equable while the intermediate furrow has a little channel; a cast 

 can be distinguished from an original rippled surface, by the channel 

 in the original surface producing a sharp little crest on the cast 

 ("Man. Geol.," Jukes and Geikie, p. 172, 3rd Ed.). 



The points that I have endeavoured to establish in the foregoing 

 pages may be briefly recapitulated as follows : — Marine ripplemarks 

 are formed by alternate currents set up by waves. Experiments with 

 short high waves on a small scale prove strong action at a depth of 

 half the wave-length, whilst the evidence of the marine fauna, and the 

 testimony of nautical men go far to prove that ocean and channel 

 waves strongly affect the sea bottom to at least the same relative 

 depth. The depth at which proof of wave action can be forthcoming 

 falls far short of that at which fine deposits can be rippled by currents 

 incapable of leaving permanent traces in the damage done to shells. 



It is scarcely necessary to point out that the subject treated of in 

 the present paper, namely, the formation of ripplemark, is only one 

 branch of a far wider and more important one, which for the sake of 

 brevity, I have as much as possible kept in the back ground, namely, 

 that of submarine denudation. 



II. " Note on General Duane^s Soundless Zones." By Dr. John 

 Tyndall, F.R.S. Received March 21, 1882. 



In reference to one of the powerful fog-whistles established on the 

 coast of Maine, General Duane remarks as follows : — "The most per- 

 plexing difficulties, however, arise from the fact that the signal often 

 appears to be surrounded by a belt varying in radius from 1 to H 

 mile from which the sound appears to be entirely absent. Thus, in 

 moving directly from a station the sound is audible for the distance of 

 a mile, is then lost for about the same distance, after which it is again 

 distinctly heard for a long time. This action is common to all ear- 



* For an illustration of the former class see plate facing page 170 of "The 

 World's Foundations," by Miss Agnes Giberne ; and, for an illustration of the 

 latter, see page 19 of Sir Charles Lyell's " Elements of Greology," 6th Edition. 

 A.R.H., 4th May, 1882. 



