CORAL FORMATIONS. 621 



enough for ship-navigation, and occasionally, as off eastern Aus- 

 tralia, fifty or sixty miles wide. On the other hand, they may be 

 too shallow for boats ; in which case the barrier-reefs coalesce with 

 the fringing reefs. 



The barrier sometimes becomes wooded for long distances, like 

 the reef of an atoll ; but usually the wooded portion, when there 

 is any at all, is confined to a few islets. 



The barrier and fringing reefs are formed precisely like the atoll 

 reefs ; and special explanations are needless. 



The absence of reefs from parts of coasts of islands within coral-reef seas 

 is due to several causes : — (1) to the depth of water, for corals fail if the depth 

 exceeds one hundred feet; (2) to fresh-water streams, especially if bringing in 

 detritus, which destroys the living corals ; as such fresh waters flow over the 

 surface of the salt, they do not prevent the corals from growing below, \inless 

 impure with detritus; (3) tidal and other currents which keep passages open, 

 by means of the detritus they often bear along their course. These are the 

 principal causes that prevent the harbors from becoming filled with corals and 

 thereby destroyed. 



The growth of the different parts of a reef, or its prolongation in one direc- 

 tion or another, depends much on the tidal and other currents that sweep 

 through the channel or by the side of the island. As in the case of silt along 

 other sea-shores, the coral detritus made by the waves is distributed by these 

 currents ; and hence the increase of a reef is not dependent solely on the number 

 of growing corals over its surface, or their kinds. 



Breadth of reefs. — The reefs adjoining lands have sometimes great 

 width. On the north side of the Feejees the reef-grounds are five to 

 fifteen miles in width. In New Caledonia they extend one hun- 

 dred and fifty miles north of the island, and fifty south, making a 

 total length of four hundred miles. Along northeastern Australia 

 they stretch on, although with many interruptions, for one thou- 

 sand miles, and often at a distance, as just stated, of fifty or sixty 

 miles from the coast, with a depth between of fifty or sixty fathoms. 

 But the reefs as they appear at the surface, even over the widest 

 reef-grounds, are in patches, seldom over a mile or two broad. 

 The patches of a single reef-ground are, however, connected by the 

 coral basement beneath them, which is struck, in sounding, at a 

 depth usually of ten to forty or fifty feet. 



The transition in the inner channels from a bottom of coral detri- 

 tus to one of common mud or earth, derived from the hills of the 

 encircled island, is often very abrupt. Streams from the land bring 

 in this mud and distribute it according to their courses through 

 the channels. 



Thickness of reefs. — The thickness of a coral formation is often 

 very great. From soundings within a short distance of coral 



