100 ON CORAL REEFS AND ISLANDS. 



The coast to the north, as far as latitude 21° N., is within the 

 warm limits, hut without reefs. In Captain Colneit's voyage, 

 allusion is made to a beach of coral sand on one of the Revilla- 

 gigedo Islands, in latitude 18° ; beside this statement, I have 

 met with no allusion to corals on any of the islands off the Mexi- 

 can coast. The paucity of corals in this region may perhaps be 

 owing, in some degree, to the fact that the tropical currents of 

 the ocean flow westward instead of eastward ; and, consequently, 

 they prove an obstable to the distribution of polyps to this coast 

 from the islands of the Pacific. Moreover, the cold currents 

 which pass the Galapagos form an impassable barrier between 

 the Paumotus and Mexico. 



Between the South American coast and the Paumotus are two 

 rocky islands, Easter or Waihti, and Sala-y-Gomez, both of 

 which are without reefs.* 



The Paumotus commence in longitude 130° W., and embrace 

 eighty coral islands, all of which, excepting about eight of small 

 size, contain lagoons. Besides these, there are, near the south- 

 ern limits of the archipelago, the Gambier Islands, and Pitcairn, 

 of basaltic constitution. The former, in 23° S., have extensive 

 reefs; about the latter, in 25° S., there are some growing corals, 

 but no proper reefs. 



The Marquesas, in latitude 10° S., have but little coral about 

 them ; and this is the more remarkable, as they are in close prox- 

 imity to the Paumotus. But their shores are, in general, very 

 abrupt, with deep waters close to the rocks. An island which, 

 before subsidence has commenced, has some extent of shallow 

 waters around, might have very bold shores, after it had half 

 sunk beneath the waves. This would be the case with the 

 island of Tahiti; for its mountain declivities are, in general, sin- 

 gularly precipitous, except at base. The Marquesas may, there- 

 fore, have once had barrier reefs, which were sunk from too 

 rapid subsidence; and afterwards, on the cessation of the subsi- 

 dence, others failed to form again, on account of the deep waters. 



The Society Islands have extensive coral reefs, with distant 

 barriers. The reefs of Tahiti extend, in some parts, a mile from 

 the shores. Tethuroa. to the north of Tahiti, and Tubuai, near 

 Bolabola, are lagoon islands. Maitea, east of Tahiti, is a sugar- 

 loaf truncated at. summit, four miles in compass, and is said by 

 Forster to have an encircling reef.f 



South of the Society Islands, near 25° S., is Rapa, which is 

 represented as a collection of rugged peaks without coral shores. 

 The Rurutu and Hervey Islands, just northwest of Rapa, have 

 coral reefs fringing the shores. There is no evidence of recent 



* Captain Beechey mentions that at forty-one fathoms, near Sala-y-Gomez, he 

 found a bottom of sand and coral, 

 f Darwin, op. cit., p. 153. 



