148 DYNAMICAL GEOLOGY. 



tends by its escape to keep one or more passages open, which, when suffi- 

 ciently deep, make entrances for shipping. 



2. Coral Beefs. 



The coral reefs around other lands or islands rest on the bottom along 

 the shores. They are either fringing or barrier reefs, according to their 

 position. Fringing reefs are attached directly to the shore, while barrier 

 reefs, like artificial moles, are separated from the shore by a channel of 

 water. The island represented in Fig. 147 has a fringing reef (/), and a 

 barrier reef (6) with an intervening channel. To the right of the middle 

 the reef is wanting, because of the depth of water ; and, farther to the right, 

 there is only a fringing reef. Fig. 149 is a map of an island with a fringing 

 reef ; and Figs. 150-152, others, with barrier reefs. At two points through 

 the barrier reef, in Fig. 147, there are openings to harbors (h). The chan- 

 nels from harbor to harbor around an island are sometimes deep enough 



147. 



..4-'yf|V>^^ 



View of a high island with baiTier and fringing reefs. 



for ship navigation, and occasionally, as ofE eastern Australia, fifty or sixty 

 miles wide ; but they are generally too shallow for boats. The barrier some- 

 times becomes wooded for long distances, like the reef of an atoll ; but 

 commonly the wooded portion, if any exists, is confined to a few islets. 

 Barrier and fringing reefs are formed like atoll reefs ; and special explana- 

 tions are needless. 



The reefs adjoining lands have sometimes great width. On the north 

 side of the Fijis, the reef -grounds are five to fifteen miles in width. In 

 New Caledonia, they extend 150 miles north of the island, and 50 south, 

 making a total length of 400 miles. Along northeastern Australia, they 

 stretch on, although with many interruptions, for 1000 miles, and often 

 at a distance, as just stated, of 50 or 60 miles from the coast, with a depth 

 of 300 or 360 feet between. 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 below 

 by coral rock, which is struck, in sounding, at a depth usually of 10 to 40 

 or 50 feet. 



The transition in the inner channels, from a bottom of coral detritus 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. 



