J. D. Dana — Origin of Coral Reefs and Islands. 189 



-atolls of middle and larger size commonly have one third to 

 two-thirds of the encircling reef covered with the sea at one- 

 third tide, making the ocean and lagoon for more than half 

 the time continuous, the large lagoon in such a case has as 

 pure water as the ocean, and commonly as good a supply of 

 food-life, and sometimes as brilliant a display of living cor- 

 als. But in the smaller atolls, the area of the lagoon has little 

 extent compared with the length and area of the encircling reef; 

 coral sands and other calcareous material consequently have pos- 

 session of the larger part of the bottom, and the waters, since 

 they are less pure than those outside, contain fewer and hardier 

 kinds of corals and less life of other kinds. They are exposed, 

 also, to wider variations of temperature than the outer, with in- 

 jury to many species, and at lowest tides may become destruc- 

 tively overheated by the midday sun, as many a plantation of 

 corals with dead tops for a foot or more bears evidence. In 

 the smallest atolls, the lagoons are liable also to alternations of 

 excessive saltness from evaporation and excessive freshness 

 from rains, and consequently no corals can grow inside, though 

 still flourishing well in the shallow sea about the outer reef. 

 The above are the facts, not the suggestions of theory. 



i. We read : " So great is the destructive and transporting 

 influence of the sea under the combined or antagonistic work- 

 ing of tides, currents and wind-waves that the whole mass of 

 the reef, as well as the flats and shoals inside, may be said to be 

 in more or less active movement."* This description of the 

 Tortugas reefs is not applicable to the atolls of the Pacific. 

 Notwithstanding the testimony of Captain Beechey and others 

 about occasional catastrophes — which are mostly catastrophes 

 to the islets and banks within the lagoons — I was led to look 

 upon a coral island as one of the most stable of structures. 

 The waves and currents have shaped its reef, shore platform, 

 and beaches, fitting it well in all respects for its place by means 

 of the forces that were to assail it ; and an air of placid repose, 

 as it lies amid the breakers, is its most impressive feature. 

 Through the wind-made and tidal movements, the loose sands 

 are drifted along the shores and over the reef ; edges of the 

 reef are broken off in gales or by earthquake waves ; and occa- 

 sionally a mushroom islet in the lagoon, where growing coral? 

 are not compacted by wave-action, is overthrown by the same 

 means; but beyond this the structure is singularly defiant of 

 the encroaching waters. Earthquakes may bring devastation ; 

 and so they may to other lands. 



* Address, p. 23. 



