On Coral Reefs and Islands. 341 
The coast to the north, as far as latitude 21° N., is within the 
warm limits, but withont reefs. In Captain Colnett’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 ma 
owing; in some degree, to the fact that the tropical currents of 
e ocean flow westward iustead of eastward ; and, consequently, 
they prove an obstable to the distribution of polyps to this coast 
from the islands of the Pacific. Mor reover, 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 Waihu, 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 — But their shores are, in general, very 
ersiin, with deep waters close to the rocks. An island which, 
idence os pene te some extent of shallow : 
nas 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- 
—* precipitous, except at base. ‘The Marquesas may, there- 
, have once had barrier reefs, which were sunk from too 
oad 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 _ of Tahiti extend, in some parts, a mile from 
the shores. - 'Tethuroa, to the north of Tahiti, and Tubuai, near 
Bolabola, are laces islands. Maitea, east of Tahiti, isa sige 
loaf truncated at summit, four miles in compass, and is said b y 
Forster to have an encircling reef. 
South of the Society Islands, near 25° S., is Rapa, erhtich is 
represented as a ae of rugged peaks without coral shores. 
The Rurutu and Herv y Islands, just northwest of Rapa, have 
coral reefs fringing the ne There is no evidence of recent 
* Captain Beechey mentions that at forty-one fathoms, near Sala-y-Gomter, he 
found a bottom pe ag coral, 
p. 153, 
