﻿THE CURRENTS OF THE OCEAN. - 135 



The action of the waves upon beaches creates the ribbed surface, the curvilinear out- 

 line, and apparent stratification which they exhibit. The light sea-weed, carried to the 

 highest reach of the tide, gathers about it and along its extent the sand that is trans- 

 ported, either by the water or the wind. The curved lines follow the natural form of the 

 summit of the wave ; and the ribbed surface is an alternation of small sand-ridges and 

 pebbly hollows, attributable to the abrasion of the latter, rolling back in a heap with the 

 receding motion of the wave. 



But the most noted and interesting effect of waves is the ripple-mark, seen upon the 

 beaches of lakes as well as of the sea. It is produced under water by that slight agita- 

 tion of the surface from which it takes its name, and is most conspicuous, perhaps, in 

 beaches of long and gentle slope. Shores of this description are common to lakes, where, 

 accordingly, the ripple-mark is abundantly displayed.* But the beaches of the sea are 

 very irregular and various in declivity in the region of tides, as might be expected from 

 the deposits which it is their office to make ; and the same portions of the shores are 

 alternately covered and exposed as the tides flow and ebb. Ripple-marks, therefore, will 

 not be distributed over the beaches of the sea as uniformly as over those of lakes. Being 

 similar in both instances, they must be distinguished from each other in the earlier geo- 

 logical periods by geographical positions. The ripple-mark is visible on the surface of 

 many sandstone deposits, and may be taken, according to Mr. Agassiz, as a guide for 

 estimating the early action of the moon upon the earth's surface, by means of the 

 tidal motions. 



In many places the sea-beaches are covered with longitudinal ridges of coarse gravel, 

 pebbles, and stones, rising sometimes to a considerable height, lying parallel to the course 

 of the shore, and following its sinuosities. A very interesting example of this is pre- 

 sented in Chelsea Beach, in Boston harbour. The ridge begins at the entrance of the 

 bay, where it attains the greatest height, and where the stones, unmixed with gravel or 

 sand, are of the largest size. The height and the size of the stones diminish gradually 

 towards the bottom of the bay, until the pile terminates in a small ridge of gravel and 

 sand, without stones. This phenomenon results from the combined action of waves and 

 currents. At the upper part of the beach, there are high cliffs of coarse drift, which are 

 washed and undermined by the sea, and their debris is carried away by the tidal currents 

 in the direction of the flood, that is, towards the upper part of the beach. During violent 

 storms, to which the beach is exposed, the coarse materials are projected upon the shore 



* As the memoir and chart of M. A. Guyot have made us intimately acquainted with the lake of Neufchatel, 

 I will adduce the southern shore of that lake as an obvious example. 



