82 Couthouy on Coral Formations. 



Lowell Institute, in this city, by that eminent Geologist, Prof. 

 Charles Lyell, a part of his language, while describing these 

 Paiimotus or Attols, was calculated to mislead many of his au- 

 dience as to their general configuration. He invariably spoke 

 of them as ''circular," "annular," or ''ring-shaped," and they 

 were so represented in the drawings illustrative of his re- 

 marks. Indeed, the question has since been more than once 

 put to me, how was this uniformity of outline to be accounted 

 for, unless the reefs really were based on submarine craters ? 



But so far from this particular shape being the constant or 

 even most frequent one, it is of comparatively rare occurrence, 

 at least in the Polynesian seas. The most ordinary form is 

 that of a short bow, crescent, or horseshoe ; the convex side 

 facing different points of the compass in different islands. In 

 those of the Dangerous Archipelago, a very common figure is 

 a long, narrow, sinuous ellipsis. This, indeed, is the config- 

 uration one might expect a group of these Paumotus to 

 assume, following that of the pre-exi stent ridges whose site 

 they occupy. 



Unassisted by plans or sections, it is not easy for the mind 

 to follow out the appearances that would be presented by a 

 mountainous tract surrounded by a shore reef, during its 

 transition to the lagoon formation. Nevertheless some notion 

 of this may be formed, if we imagine to ourselves an island 

 like Tahiti or Eimeo, or some of those in Samoa, consisting 

 of a number of central conical peaks, (some of them crater- 

 iferous,) from which diverge in all directions, sharp ridges 

 having upon them, here and there, hills sometimes several 

 hundred feet high — these ridges intersected by profound 

 ravines, whose walls frequently present a precipice of fifteen 

 hundred or two thousand feet elevation — and the ravines 

 sometimes barred by a transverse ridge, perhaps a portion 

 of the mountain, which has fallen down, so as to give the 

 space between the barrier and the head of the chasm the ap- 

 pearance of a long, narrow, and deep pit or trench. 



The stupendous ravines which separate the lateral ridges of 

 the central chains, form such a remarkable feature in all the 

 volcanic islands of Polynesia, that they seem to me entitled 



