CAUSES MODIFYING THE FORMS AND GROWTH OF REEFS. 79 



2. Causes modifying the Forms and Growth of Reefs. 



Coral reefs although (1) dependent on the configuration of the 

 submarine lands for many of their features, undergo various 

 modifications of form or condition through the influence of ex- 

 traneous causes, such as (2) unequal exposure to the waves ; (3) 

 oceanic or local currents ; (4) presence of fresh or impure waters. 

 In briefly treating of these topics, we may consider first, reefs 

 around high islands, and afterwards, atoll reefs. The effect of the 

 waves on different sides of reefs has already been considered, and 

 we pass on, therefore, at once to the influence of oceanic or local 

 currents, and fresh or impure waters. 



a. Barrier and Fringing Reefs. — The existence of harbors 

 about coral-bound lands, and of entrances through reefs, is largely 

 attributable to the action of tidal or local marine currents. The 

 presence of freshwater streams has some effect towards the same 

 end, but much less than has been supposed.* 1 



There are usually strong tidal currents through the reef chan- 

 nels and openings. These currents are modified in character by 

 the outline of the coast, and are strongest wherever there are 

 coves or bays to receive the advancing tides. The harbor of 

 Apia, on the north side of Upolu, affords a striking illustration of 

 this general principle. The coast at this place has an indenta- 

 tion 2000 yards wide and nearly 1000 deep, as in the accompany- 

 ing sketch, reduced from the chart by the 

 Expedition. The reef extends from either 

 side or cape a mile out to sea, leaving 

 between, an entrance for ships. The har- 

 bor averages ten feet in depth, and at the 

 entrance is fifteen feet. In this harbor %!&> 

 there is a remarkable out-current along the 

 bottom, which during gales, is so strong 

 at certain states of the tide, that a ship at 



i i.i i -i i ii HARBOR OF APIA, UPOLU. 



anchor, although a wind may be blowing 



directly in the harbor, will often ride w 7 ith a slack cable ; and in 

 more moderate weather the vessel may tail out against the wind. 

 Thus when no current but one inward is perceived at the surface, 

 there is an under current acting against the keel and bottom of the 

 vessel, which is of sufficient strength to counteract the influence 

 of the winds on the rigging and hull. The cause of such a cur- 

 rent is obvious. The sea is constantly pouring water over the 



* The view here supported, is nearly identical with that presented by Mr. Dar- 

 win, (op. cit, page 68.) The arguments given were, however, written out (in 1840), 

 before his descriptions of coral islands were known to me. This fact may give ad- 

 ditional weight to the opinions, inasmuch as they are therefore the conclusions of 

 independent observers, and are substantiated by a distinct set of observations. 



■Haga 



