CORAL FORMATIONS. 619 



others, one side is wooded continuously, or nearly so, while the other 

 is mostly bare, oris a string of green islets, as in Fig. 961, representing 

 Apia, one of the Gilbert Islands. The higher or wooded side is that 

 to the windward, unless it happens to be under the lee of another 

 island. On the leeward side, channels often open through to the la- 

 goon (e, Fig. 961), which, when deep enough for shipping, make the 

 atoll a harbor ; and some of these coral-girt harbors in mid-ocean are 

 large enough to hold all the fleets of the world. 



Fig. 963 represents a section of an island, from the ocean (o) to the 

 lagoon (I). On the ocean side, from o to a, there is shallow water for 

 some distance out (it may be a quarter or half a mile or more) ; and, 

 where not too deep (not over one hundred feet), the bottom is covered 

 here and there with growing corals. Between a and b there is a plat- 

 form of coral rock, mostly bare at low tide, but covered at high, having 



Fig. 963. 



Section of a coral island, from the ocean (o) to the lagoon (I). 



a width usually of about a hundred yards : there are shallow pools in 

 many parts of it, abounding in living Corals of various hues, Actiniae 

 (Sea-anemones), Star-fishes, Sponges, Shells, Shrimps and other kinds 

 of tropical life: toward the outer margin, it is quite cavernous ; and the 

 holes are frequented by Crabs, Fishes, etc. At b is the white beach, six 

 or eight feet high, made of coral sand or pebbles and worn shells : b 

 to d is the wooded portion of the island. The whole width, from the 

 beach (b) to the lagoon (c), is commonly not over three or four hun- 

 dred yards. At c is the beach on the lagoon side, and the commence- 

 ment of the lagoon. Corals grow over portions of the lagoon, — 

 although, in general, a large part of the bottom, both of the lagoon 

 and of the sea outside, is of coral sand. 



Beyond a depth of one hundred feet, there are no growing corals, 

 except some kinds that enter but sparingly into the structure of reefs, 

 the largest of which are the Dendrophyllioe. 



Coral-reef Rock. — The rock forming the coral platform and other 

 parts of the solid reef is a white limestone, made out of corals and 

 shells. Its composition is like that of ordinary limestones. 



In some parts, it contains the corals imbedded ; but, in others, it is 

 perfectly compact, without a fossil of any kind, unless an occasional 

 shell. In no case is it chalk. The compact non-fossiliferous kinds 

 are formed in the lagoons or sheltered channels ; the kinds made of 

 broken corals, on the seashore side, in the face of the waves ; those 



