810 STUDIES FOR STUDENTS 



delicately dissected by rill marks, an example of this phenomenon 

 having been described by Dodge. 1 



Among the earlier accounts of ripple marks, one of the most 

 interesting is based on the little-known work of an ingenious 

 French engineer named Siau. 2 In 1841 he published a brief note 

 entitled "De Taction des vagues a de grandes profondeurs," based 

 on observations of ripple marks in deep water, made with the aid 

 of an ordinary sounding apparatus. While examining ripple 

 marks, visible during quiet water, on the bed of a channel off the 

 west coast of the Isle of Bourbon, Siau noted that the heavier 

 particles of the sand tended to accumulate in the troughs between 

 the ridges, while lighter material was concentrated along the ridge 

 crests. Profiting by this discovery, he coated a sounding lead 

 with tallow and lowered it to the sea floor where the depth was too 

 great for direct visual observation. When brought to the surface 

 the tallow sometimes retained, adhering to it, only heavy particles 

 of sand, in which case the surface of the tallow had a convex form, 

 showing that it had been pressed down into the trough between 

 two ripples. In other cases the tallow was coated with lighter 

 particles only, and had a concave form, as a result of having been 

 pressed down upon a ripple crest. At great depths, where the 

 ripples were more closely spaced, two parallel bands of materials 

 differing in specific gravity would be impressed upon the tallow 

 at the same time, the heavier material coating a convex ridge, and 

 the lighter a concave depression in the tallow. By this ingenious 

 device Siau was able to prove the existence of ripple marks at 

 a depth of 617 feet. 



The ripples described by Siau were believed by him to be due 

 to the back-and-forth currents which are produced on a sea bottom 

 by oscillatory waves. Such ripple marks are called "oscillation 

 ripples," and are characterized by symmetry of crests, neither slope 

 being steeper than the other, since the ridges are built up by cur- 

 rents which operate from both sides with approximately equal 



1 R. E. Dodge, "Continental Phenomena Illustrated by Ripple Marks," Science, 

 XXIII (1894), 38-39. 



3 Siau "De Taction des vagues a. de grandes profondeurs," Comptes rendus de 

 I'academie des sciences, XII (1841), 774, and Annates de chimie et de physique, 3® Ser. 

 (1841), 118. 



