540 



PROCEEDINGS OF THE ACADEMY OF 



[Nov., 



to the next crest, and this gentle slope having its upper part contin- 

 ually cut away by the water action, making a little subsidiary ripple- 

 mark (fig. 2). During the advance of the tide these ripple-marks 

 are formed over large strand surfaces, and when the tide recedes 

 they are often modifed and the troughs deepened by erosion. In 

 some cases they are obliterated, in other cases they may be covered 

 up by other sand deposits and preserved; but in general this last 

 will only happen occasionally, when an exceptionally high tide or a 

 flood from a river brings down an unusual amount of material from 



f.-i" 





rf-Nl 





Fig. 3. — View of rippled strand. 



the shore. As the streams here are mostly bog-fed, this can only 

 happen by landslide or other cause that adds a large amount of 

 detritus to the stream water. At Dove Point the ripples in the 

 sand of the strand could hardly be preserved, except in case of very 

 high tide. This double form of ripple of deposition is found in wind- 

 swept sand also, but does not seem to be very common among the 

 ripple-marked surfaces that are preserved in the rocks. It is quite 

 probable that the bulk of these are produced by erosion alone, not 

 by deposition, and the ripples made by erosion are of a different 

 form. 



