622 



DYNAMICAL GEOLOGY. 



History of the emerging Atoll. — The growing corals and the accu- 

 mulating debris reach, at last, low-tide level. The waves continue to 

 pile up on the reef the sand and pebbles and broken masses of coral, 



— some of the masses even two or three hundred cubic feet in size, 



— and a field of rough rocks begins to appear above the waves. Next, 

 a beach is completed ; and the sands, now mostly above the salt water, 

 are planted by the waves with seeds ; and trailing shrubs spring up : 

 afterward, as the soil deepens, palms and other trees rise into forests ; 

 and the atoll comes forth finished. 



The windward side of such islands is the highest, because here the 

 winds and waves act most powerfully ; and, where the leeward side of 

 one part of the year is the windward of another, there may not be 

 much difference between the two. The water that is driven by the 

 winds or tides over the reef, into the lagoon, tends, by its escape, to 

 keep one or more passages open, which, when sufficiently deep, make 

 entrances for shipping. 



2. Coral Reefs. 



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. 



Fig. 964 represents an island with a fringing reef (/) a barrier 

 reef (b), and an intervening channel. Just 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. 966 is a map of an island 



View of a high island with barrier and frit)giDg reefs. 



with a fringing reef; and Figs. 967-969, others, with barrier reefs. At 

 two points through the barrier reef, in Fig. 964, there are openings to 

 harbors (h). Such harbors are common, and generally excellent. The 

 channels uniting them around an island are sometimes deep enough 

 for ship navigation, and occasionally, as off eastern Australia, 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, 



