542 PROCEEDINGS OF THE ACADEMY OF [Nov., 



As the tide rose continuously, and the water advanced regularly 

 over the flat, there was a progressive flow of the water over the 

 surface of the mud; but, in some places, ripples were regularly 

 produced; in other places, especially near the shore, the surface was 

 nearly plane and free of ripples, Plate XLI, fig. 2. On these surfaces 

 other markings were produced. The rippling of the water by the 

 winds is the cause of the formation of these erosion ripples. Their 

 formation was investigated by wading about in the shallow water, 

 two to six inches deep, while the tide was advancing over the flats. 

 It was then seen that at the passage of each wave there was raised 

 a slight cloud of muddy water from the concave of the ripple. The 

 surface of the clay was softened for about one or two millimeters 

 in depth when the tide was in over the flats, and in this soft surface 

 impressions could be made by any object moving over the bottom 

 and touching it, from time to time or continuously. But the ripples 

 were very permanent, remaining in the same place for days. That 

 they have the origin assigned to them there can be no doubt, they 

 are not of the form of the ripples of deposition and they are firm 

 and persistent. They closely resemble the ordinary forms of ripples 

 that are seen on the rock surfaces in the Trias, for example; and 

 that erosion was the source of these Triassic ripples in many cases, 

 I have very little doubt. They are very constant in form, as has been 

 noted, and very persistent, owing to the tough and tenacious char- 

 acter of the Leda clay. That such ripple-marked surfaces should 

 be preserved if the flats were subject to inundations by water highly 

 charged with sediment seems certain; and while such was not the 

 case here, the water would be just as efficient as an erosive agent 

 if it contained a charge of sediment. The wind waves start the 

 erosion of the ripple; but, as it develops, it reacts upon the water, 

 producing waves when the wind is not blowing as well as when the 

 surface of the water is rippled by the wind. That this form of 

 erosion ripples is more common on hard mud bottoms than the 

 ripple of deposition and that on sandy bottoms the ripple produced 

 by deposition is more common, is indicated, too, by an examination 

 of rippled surfaces of rock. The finer material seems to be more 

 often marked with the symmetrically concave, sharp crested, erosion 

 ripples; while the coarser materials, such as sands, show more often 

 the rounded crest and peculiar unsymmetrical shape of the deposi- 

 tion ripples. 



The Tracks of Animals.- — As has been noted above, large mammals 

 and birds, moving over the clay flats, left their foot-marks upon the 



