46 DYNAMICAL GEOLOGY. 



probably not exaggerated, in what is said above, the 

 ravages it has suffered from its ancient enemy, the 

 I sea. 



Transportation and Deposit. The lifting power of 

 waves is immense, often taking up rock-fragments of 

 many tons weight and hurling them with violence against 

 the shore-line ; but they usually carry only a very short 

 distance. Under certain conditions, however, waves 

 may transport materials for many hundreds of miles. 

 Thus, on account of the trend of the Atlantic coast and 

 the prevalence of north winds, the coast material is cast 

 up on shore and falls off a little southward with every 

 wave. Thus, shore-sands creep southward slowly, even 

 to the point of Florida, although the coast-rock of Flor- 

 ida is all limestone. So, also, the shore-sands of Lake 

 Michigan are carried southward by wave-action, and 

 accumulate about Chicago. 



Though waves are usually destructive rather than con- 

 structive, yet they often add to the land along shore- 

 lines by deposits. Such deposits are very characteristic : 



1. They are usually coarse material and thoroughly 

 water-worn i. e., round-grained sand, gravel, or shingle. 



2. The lamination is often highly inclined and irregular. 



3. They are often affected with ripple-marks (Fig. 24). 



FIG. 24. Ripple-marks. 



4. They are often impressed with tracks of animals and with 

 rain-drops. Now, all these marks are found in rocks far 

 inland and high up the slopes of mountains. We can thus 



