74 Couthouy on Coral Formations 



was shown by a registering thermometer to be but 56°, and 

 at nine hundred feet but 48°. This experiment was made 

 by myself, in the open ocean. At the same depths upon 

 soundings, the difference would, I doubt not, have been still 

 greater, but not having actually ascertained this by experi- 

 ment, I cannot speak positively on this point. 



According to the statements of those zealous naturalists, 

 MM. Quoy and Gaimarcl, the result of their observations during 

 the first voyage of the Astrolabe, was, that the growth of the 

 more solid corals was limited to a depth of five or six fathoms.* 

 In fixing this limit, however, 1 think they have not suffi- 

 ciently taken into consideration the variations of temperature 

 at small depths, produced by accidental causes, and that in 

 the tropics, where the sea is warmed to a considerable depth 

 by the presence of large bodies of land, these corals may 

 flourish considerably lower. 



In approaching the island of Tutuila, one of the Samoan 

 group, I remember suddenly coming from deep water upon a 

 shelf, upon which there were but thirteen fathoms. This 

 ledge, distant about two and a half miles from the coast, 

 which was very steep, was profusely covered with coral. The 

 surface temperature was here 81°, and that of the bottom 76°, 



* The work of Q,, and G. not being accessible here, I trust of necessity to 

 memory, in quoting the depth assigned hy them, as the lowest limit for the 

 growth of the coral in any considerable quantity. 



It is well perhaps to notice here, that wherever, in this communication, cer- 

 tain depths and temperatures are spoken of, as essential to the growth of the 

 polypes, I refer only to the reef-constructing genera, and more particularly to 

 those whose Polyparia form hemispheral masses, broad, lamellar incrustations, 

 or solid palmate clusters. Some of the arborescent corals have been found in 

 extra-tropical seas, in very low temperatures, and depths far exceeding those 

 here assigned as the limits of the saxigenous polypes. There is now, or should 

 be, in the collection at Washington, a small species of Madrepora, dredored on 

 the coast of Patagonia, from a depth of eighty fathoms; and Dr. Gould has 

 lately received specimens of another from our coast, in the vicinity of Portland, 

 M;iine. I have also picked up specimens on the New Jersey shore. But these 

 have all a shrunken, dull, and if I may so call it, starved appearance, and are of 

 insignificant size. Such species may, and I doubt not do, exist at depths of 

 corresponding temperature, in the tropics, but they bear the same aflRnity to 

 those constituting the coral reefs, that our humble bracken does to the magnificent 

 and stately palm-tree-like Ferns of Polynesia. 



