34 SEA AND LAND 



the streams have a swiftness and energy comparable to those 

 exhibited by the greater cataracts. The capacity of the tidal 

 currents, like that of all streams, their power to scour and 

 convey sediments, depends immediately on the speed with 

 which they move. When, as on the eastern coast of Maine, 

 they often flow at the rate of six or eight miles an hour, 

 a speed nowhere attained by the waters of the Mississippi, 

 they strip all the shores on which they impinge of all their 

 fine detritus which may have accumulated there, and thus 

 expose the rock to the effective action of the waves. 



For the reason that these tidal currents are most energetic 

 when they are confined as in a wedge-shaped bay, they exert 

 their maximum influence not on the open coast, but in the 

 recesses of the shore. The waves of the ocean tend to force 

 the detritus the)- have torn from the exposed part of the 

 shore into every neighboring bay, thus in time destroying 

 all the inlets and bringing the shore to a uniformity of 

 outline; but where the sun and moon pull the waters about 

 and send them whirling into the bays and harbors, the cur- 

 rents which are thereby produced scour out the sand, clay, 

 and pebbles which the waves have imported into these 

 recesses and remove them again into the o\iQ.\'\ sea. 



Along every continental shore, and often far into the 

 interior of these great land masses, we may discern indications 

 that the work which we now observe to be going on upon 

 the coast-line has at various times been effected at levels 

 far above the present plane of action. These ancient sea 

 margins, as they are termed, have been noted by observers 

 since the early part of this century. It is only, however, 

 within the last fifty years that geologists have begun to 

 attend to this class of phenomena. A careful study of such 



