1882.] 



On the Formation of Ripplemark. 



17 



reduce the number of the low waves to between 80 and 90 per minute. 

 These, though barely over \ of an inch in height, immediately affected 

 the submerged vane, and the motion of the index attracted my eye, 

 though I was not attending to it. On a subsequent occasion I found 

 that waves between 80 and 90 per minute, and only ^ of an inch high, 

 moved the index at a depth of 6J inches, and moved flocculent matter 

 on the sandy bottom at a depth of 7| inches. 



In this experiment motion at the bottom was obtained when the 

 depth was 62 times the height of the wave,, though small in propor- 

 tion to the wave-length. 



If the evidence of the existence of alternate currents on the floor of 

 shallow seas is strong; the evidence that the currents of power 

 sufficient to roll and damage shells, are not constant currents, is still 

 stronger. Molluscs, such as- pinna*, and mya, that have little, if any, 

 powers of locomotion, could not coexist with currents capable of 

 transporting in any quantity, even fine sand. They would perish, 

 either from the destruction of their beds by the removal of the sand, 

 or from fresh material being piled on top of them. Mr. Godwin 

 Austen has well, said, that a " drift-sand zone " is wholly unfitted for 

 marine life.* But it is the drifting sand that is fatal, not the mere 

 fact that the drift sand zone is the one that " comes within the range 

 of the tidal and wave disturbance of the water." Off Paignton Sands 

 in Torbay, the very zone described by Mr. Godwin Austen abounds in 

 Gardium tuberculatum and Donax vittatus, but owing to the fact that 

 the sands are open to heavy seas from one quarter only, they cannot 

 drift, and the shells mentioned,, as^ a rule survive the attacks of the 

 waves, though many individuals succumb. 



As ripple marks are formed under water, so also they can be pre- 

 served under water ; and they are more likely to be there preserved than 

 on a sea beach, where on the retreat of the tide, they are liable to be 

 effaced by the very swell that has formed them. 



In the case of a lake or sea, subject to slight changes of level, with 

 a river running into it carrying mud or sand, we have all the con- 

 ditions necessary for the formation and preservation of ripplemark. 

 An occasional swell from the lake will ripple the submerged sand in 

 the vicinity of the accumulating deposits brought down by the stream, 

 and these will quietly cover up the ripples without effacing them. A 

 slight rise in the level of the lake will shift the area of deposition 

 further up the stream, and the bed that covered the ripples will in 

 course of time be itself rippled, and in its turn covered up. In this 

 manner a series of ripplemarks and beds of mud or sand often " false- 

 bedded," may be rapidly formed, and in the case of increase of current 

 as rapidly destroyed. 



Ripples that have been left bare by the retreating tide, or possibly 

 * " Natural History of the European Seas," p. 253. 



VOL. XXXIV. C 



