256 PHYSIOGRAPHY. [chap. 



this zone, they are not found in all parts of it. They 

 are absent, for example, on the western coasts of Africa 

 and America ; and, where great rivers debouch, the sedi- 

 ment and the fresh water, which they pour into the sea, 

 interfere with the growth of the coral polypes. Moreover, 

 reef-forming corals are restricted, not only in their super- 

 ficial distribution, so as to be limited to certain latitudes, 

 but also in their vertical distribution, so as to be limited 

 to certain depths. Indeed, the needful conditions for the 

 growth of the polypes are found only in comparatively 

 shallow water. From the observations of Mr. Darwin, it 

 appears that these corals do not flourish at greater depths 

 than between 20 and 30 fathoms, and are, for the most part, 

 restricted to about 1 5 fathoms of water. Knowing this, it 

 might not unnaturally be assumed that coral-reefs and coral- 

 islands would always be confined to shallow seas. As a 

 matter of fact, however, soundings outside a barrier-reef, 

 or an atoll, often show an enormous depth of water, the 

 outer edge sinking down abruptly like a coral-wall. The 

 early navigators knew that coral-islands were not unfrequently 

 surrounded by very deep water ; but this fact presented no 

 difhculty, until naturalists became aware of the small vertical 

 range to which the living corals are limited. Various 

 attempts were then made to reconcile the two apparently 

 opposed facts ; but no satisfactory explanation was given 

 until Mr. Darwin, about forty years ago (in 1837) advanced 

 a most ingenious hypothesis, which not only perfectly solved 

 the puzzle, but brought the several classes of coral-reefs into 

 close relation with each other. 



According to Mr. Darwin's view, the coral-rock has, in all 

 cases, been originally formed in water not deeper than about 

 20 fathoms; and, when found at greater depths, it must have 

 been carried down by subsidence of the rocky foundation 



