2IO SEA AND LAND 



the agency of organic life. Although in the realm of the 

 tropics there are innumerable ports which owe their origin 

 to this last-named class of reefs, none of them have as yet 

 any great commercial importance. They at present serve 

 mainly for small vessels. They can in general only be made 

 useful to our modern marine by a continuous and expensive 

 process of dredging, to make head against the rapid growth of 

 the zoophyte communities and the accumulations of sediments 

 brought about by the intensely vigorous organic life which 

 develops within the coral reefs. 



The port of Hamilton on the Bermudas is perhaps the 

 best instance of an important haven situated on a coral reef 

 belonging to the class of atolls, while the beautiful Biscayne 

 Bay on the east coast of Florida is an excellent example 

 of a haven which is enclosed by barrier reefs. 



In the foregoing summary account the several classes of 

 natural actions which lead to the formation of harbors have 

 been considered. We have now to take into account the 

 other modes of operation of the geologic forces which tend 

 to alter for better or for worse the physical conditions of 

 our havens in determining the ease with which they may be 

 approached, the character of their shores, and the depth of 

 water which they afford. All these changes are brought about 

 as a result of the application of energy, operating in diverse 

 ways along our shores. Therefore the reader will the more 

 readily come to an understanding of the questions with which 

 we have to deal if we begin the inquiry by noting the gen- 

 eral nature and mode of action of these forces. 



There are four modes in which energy is applied in a 

 way to modify the conditions of coast-lines. Three of these 

 have their origin in the celestial spaces ; one only comes from 



