LIFE OF JAMES DWIGHT DANA 



are examples on record, to be found in various treatises 

 on geology, of still more surprising effects. 



" The progress of the coral formation is like its com- 

 mencement. The same causes continue, with similar re- 

 sults," and the reader might easily supply the details from 

 the facts already presented. The production of debris 

 will necessarily continue to go on : a part will be swept 

 by the waves, across the patch of reef, into the lagoon or 

 channel beyond, while other portions will fill up the 

 spaces among the corals along its margin, or be thrown 

 beyond the margin and lodge on its surface. The layer 

 of dead coral rock which makes the body of the reef has 

 its border of growing corals, and is thus undergoing ex- 

 tension at its margin, both through the increase in the 

 corals and the debris dropped among them. 



" But besides the small fragments, larger masses will 

 be thrown on the reefs by the more violent waves, and 

 commence to raise them above the sea. The clinker 

 fields of coral by this means produced constitute the first 

 step in the formation of dry land. Afterward, by further 

 contributions of the coarse and fine coral material, the 

 islets are completed, and raised as far out of the water as 

 the waves can reach that is, about ten feet with a tide 

 of three feet ; and sixteen to eighteen feet with a tide of 

 six or seven. 



4 The ocean is thus the architect, while the coral 

 polyps afford the material for the structure ; and, when 

 all is ready, it sows the land with seed brought from dis- 

 tant shores, covering it with verdure and flowers. 



" 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 fresh-water streams has some effect toward the same 

 end, but much less than has been supposed. These 

 causes are recognized by Mr. Darwin in nearly the same 

 manner as here ; yet the views presented may be taken 

 as those of an independent witness, as they were written 

 out before the publication of his work. 



* There are usually strong tidal currents through the 

 reef channels 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, 



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