200 SEA AND LAND 



yet attained in their movement between high-tide and low- 

 tide mark. At this extreme southern position the (quantity 

 of the sand is not orcat, and the ofrains of silex of which it is 

 composed are much rounded by their long and arduous jour- 

 ney, at every step of w^hich they have been beaten by the 

 waves. It appears likely that in time these wandering sands 

 w^ill attain to the extreme southern part of Florida, destroying 

 or diminishing the coral-producing polyps as they go. Prob- 

 ably to their action is due, in part at least, the diminution of 

 the reef, which is but feebly exhibited in the district to w^hich 

 this detritus has attained. 



Although sand-spit harbors or equivalent structures com- 

 posed of gravel or shingle of themselves form few impor- 

 tant havens, they are a concomitant feature in the natural 

 defences of many ports. They are often conspicuous at the 

 mouths of reentrant delta harbors, and serve to protect that 

 class of ports from the incursion of the weaves to wdiich their 

 naturally broad mouths ordinarily exposed them. This effect 

 may be clearly seen at the mouth of Mobile Bay, Galveston 

 Bay, and various other points on the Atlantic coast. Not 

 only do these spits afford protection from the ocean's surges, 

 but they often serve to deepen the channels which give access 

 to the port, and thereby to render the harbor accessible to 

 larger vessels. Where they do not exist it is often necessary 

 to accomplish the work which they might do, by means of 

 artificial construction. We may furthermore note the fact 

 that when once formed these spits tend to prevent the en- 

 trance into a bay of the sands which the weaves scour from the 

 shallowed bottom over which they roll, and which, but for the 

 presence of these barriers, might penetrate far enough into 

 the harbor seriously to reduce the depth of water. Thus the 



