194 
VOYAGE TO THE 
CHAP, of the lithophytes to fresh water is not singular, as, independent of its 
not being the natural element of those animals, it probably supplies no 
March, materials with which they can work. 
1 826 
It has been suggested, that these openings being opposite to valleys, 
the continuation of them under water is the cause of the break in the 
reef. But when we consider the narrowness of these openings, com-, 
pared with the width of the valleys, and that the latter are already filled 
up to the surface and furnished with a smooth sandy beach, many 
obstacles will be found to the confirmation of such an opinion ; and 
it appears to me more reasonable to attribute it to the nature of the 
element. The depth of these channels rarely exceeds twenty-five feet, 
the greatest limit probably to which the influence of fresh water would 
be felt. 
Henderson Island, one of the exceptions mentioned in the early 
part of this discussion, is among the rare instances of its kind in these 
seas. It is an island composed of dead coral, about eighty feet above 
the sea, with perpendicular cliffs nearly all the way round it, as if after 
being formed in the ocean it had been pushed up by a subterraneous 
convulsion. These cliffs are undermined at the base, as though the sea 
had beaten against them a considerable time in their present position. 
There are no marks upon them indicative of the island having risen 
by degrees ; but, on the contrary, a plain surface indicating its ascent by 
one great effort of nature. On examining the volcanic islands near 
Henderson Island, no traces appeared of the sea having retired ; and we 
may, therefore, presume it to have risen as described. Its length is five 
miles, and breadth one mile ; it is nearly encompassed by a reef of living 
coral, so wdde that the cliffs, which w^ere at first subjected to the w'hole 
force of the weaves, are now beyond the reach even of their spray. 
The navigation of this archipelago was made at a period of the 
year when the westerly monsoon was about to commence, and tow^ard the 
end of which it had actually begun, and materially retarded our opera- 
tions; but previous to that time, or about the beginning of March, the 
trade was fresh and steady, blowing between S. E. by E. and E. N. E., 
which is more northerly than the direction of the same trade between 
corresponding parallels in the Atlantic. In consequence of this op- 
