154 Letter X. 
the great solid structures of coral rock of which this island is 
composed, I find it very difficult to believe that these masses 
have all been drawn, grain by grain, from the ocean by such a 
minute agency. But even this island is a poor specimen of the 
magnitude of their works. Tongatabu—an island in Eastern 
Polynesia, 100 miles in circumference—is of coral formation, 
and owes its existence to the energy of these little creatures. 
Of course, countless ages must have been consumed in the 
formation of such an island as that, and countless myriads of 
builders must have been employed. But allowing time enough 
and workers enough, there is really no limit to what might be 
produced in this way. 
Mr. Darwin tells us that where coral reefs or islands exist, 
there has been or is still a gradual subsidence of the ground on 
which they stand. For the coral insects build on a bottom al- 
ways within twenty fathoms of the surface ; and as this bottom 
gradually subsides, they keep building upwards, keeping on a 
level with the surface of the ocean, where they seem best to 
thrive. In this way may be explained the great depth of 
coral reefs and the high cliffs of coral islands—merely reefs 
upheaved, which might seem to contradict the law that no 
coral insect can work at a greater depth than twenty fathoms. 
As I have said before, Aniwa ig an upheaved coral island. 
It has been formed, apparently, by two natural processes— 
first, by the foundation on which it stands gradually subsiding, 
until the coral insects built up a mass as high as the cliff is 
above the depth to which the coral now extends under the 
water ; and then, secondly, it has been upheaved. Traces of 
this great lifting force are distinctly visible on the island. 
xf VOC Duar... 
This rough section will help:me to explain what has been 
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