34 



DYNAMICAL GEOLOGY. 



ning into landlocked sons, muke deltas ; but rivers emp- 

 tying into strongly tidal seas have wide, bay-like mouths 

 or estuaries. The strong tides and waves not only carry 

 away the sediment brought down and prevent land-mak- 

 ing, but cut away and enlarge the mouths of the rivers. 

 Thus, in this country, all the rivers emptying into the 

 Great Lakes or into the Gulf of Mexico (where the tides 



FIG. 13. -Mississippi delta. 



are very small) make deltas, while all emptying into the 

 Atlantic or Pacific have estuaries. So, in Europe, all the 

 rivers emptying into the Mediterranean Sea, the Black 

 Sea, the Caspian Sea, the North Sea, and the Baltic Sea, 

 form deltas, while those emptying into the Atlantic have 

 estuaries. The Ganges (Fig. 14) seems to be an excep- 

 tion to this rule ; for it makes a great delta, although the 

 tides in the Bay of Bengal are strong. The cause of this 

 is the prodigious quantity of mud brought to the sea by 

 the Ganges. Two opposite agencies are at work at the 



