TIDAL CURRENTS AND ORGANIC LIFE 



IN HARBORS 



Effects of Tides on Harbors : Organization of Tidal Streams ; Effect of Tidal Bars. — Action 

 of Organic Life in Harbors ; Classes of Work done.— Effect of Marine Plants ; Flow- 

 ering Species ; Sea-weeds.— Growth of Marine Marshes : their Relation to Tidal Cur- 

 rents ; their Value for Tillage. — Tropical Marine Marshes. — Effect of Mangroves on 

 these Areas. — Effects of Organic Life on Harbors : Work done by Mollusks ; by the 

 Common Oyster ; Comparison of Northern and Southern Marine Marshes ; Work done 

 by other Animal Species. — Synopsis. 



The effect of tides upon a shore-line is in admirable con- 

 trast to that of waves. Although both of these movements 

 of the water are in their nature undulations, the difference 

 in their form leads to an entire contrast in their action. Thus 

 wind waves strike ordinary blows ; the tidal wave delivers 

 no stroke whatever. The surges are most effective on the 

 headlands ; the friction which the\' encounter on the bottom 

 causes them rapidly to diminish in size as they pass in between 

 the convercrine shores of an enibavment. On the other hand 

 the tidal wave, because of its breadth from front to rear, heai^s 

 up in a bay in a very remarkable manner. It may often hap- 

 pen that a recess of the shore having a length of not more than 

 twenty miles with a width at its mouth of say ten miles will 

 have a tidal rise at its head near twice as great as that at its 

 mouth. So too in larger bays as that of Fundy, a rise of tide 

 of about ten feet at its eastern cape becomes an uplift of fitty 

 feet or more at the landward end of the basin. Owing to this 



