290 Dr. J. D. Dana oti the Origin of 



accords with the reversal in the history just suggested. Since 

 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 

 corals. But in the smaller atolls, the area of the lagoon has 

 little extent compared with the length and area of the encir- 

 cling reef; coral-sands and other calcareous material con- 

 sequently have possession 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 injury to many species ; and at lowest 

 tides may become destructively 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 evapo- 

 ration 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 occasionally a mushroom islet in the lagoon, 

 where growing corals 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. 



