620 



DYNAMICAL GEOLOGY. 



History of the emerging reef. — The growing corals and the accumu- 

 lating debris reach at last low-tide level. The corals then mostly 

 die ; but 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 com- 

 pleted, and the sands, now mostly above the salt water, are planted 

 by the waves with seeds, and trailing shrubs spring up ; after- 

 wards, 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 



View of a high island with barrier and" fringing reefs. 



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. 852 represents an island with a fringing reef (/) and a bar- 

 rier (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. 854 is a map of an island 

 with a fringing reef, and figs. 855-857, others with barrier reefs. At 

 two points through the barrier reef in fig. 852 there are openings 

 to harbors (A). Such harbors are common, and generally excellent. 

 The channels uniting them around an island are sometimes deep 



