STRUCTURE OF CORAL ISLANDS. 157 



forty fathoms deep. He says that, in the words of Captain 

 Moresby,, it is in truth *' nothing more than a half-drowned atoll." 



The form of the bank, its margin of shoals, and a line 

 of soundings across it, giving the depth of the central or 

 lagoon portion, are shown in the map on ]). 155, from Darwin, 

 and for which, as well as for other information about the bank, 

 he gives credit to Captain Moresby. The cross section is still 

 further illustrated in the cut on p. 156. The whole length of 

 the section (or width of the bank in the line of the soundings) 

 is seventy-six miles. From the outer rim of the submerged 

 atoll there is a drop off to a deeper level, which is mostly 

 fifteen to eighteen fathoms below the surface ; and then to the 

 bottom of what was once the lagoon, now for the most part 

 forty to fifty fathoms under water, though having its shoals that 

 are five to ten flithoms submerged. All points in the map that 

 are shaded have a depth of less than ten fathoms ; the only 

 emerged parts are three or four spots on the western margin, 

 as indicated on the map. The bottom over the interior is 

 muddy ; on the flat bordering it, fifteen to tw^enty fathoms deep, 

 there is coral sand with " a very little live coral ; the outer rim 

 is coral rock with scarcely any live coral ; " while the shoals or 

 knolls of the interior are "covered with luxuriantly-growing 

 corals." Darwin states also that the rim is steep on both sides, 

 and outward slopes abrupdy to unfathomable depths; at a 

 distance of less than half a mile from one part no bottom was 

 found with 190 fathoms ; and off another point, at a somewhat 

 greater distance, there was none with 210 fathoms. 



Metia and other elenated Coral Islajids. — Metia, or Aurora 

 Island, is one of the western Paumotus. It is a small island 

 about four miles by two and a half in width, and two hundred 

 and fifty feet in height ; and it consists throughout of coral 

 limestone. Approached from the north-east, its high vertical 

 cliffs looked as if basaltic, resembling somewhat the Palisades 

 on the Hudson. This appearance of a vertical structure was 

 afterwards traced to vertical furrowings by the waters dripping 

 down its front, and the consequent formation of stalagmitic 

 incrustations. Deep caverns were also seen. 



