146 



DYNAMICAL GEOLOGY. 



The reef is usually to a large extent bare coral rock, swept by the waves 

 at high tide. In some reefs the dry land is eontiued to a few isolated i)oints, 

 as in Fig. 145 ; in others, one side is wooded continuously, or nearly so, while 

 the other is mostly bare, as in Fig. 144. The higher or wooded side is that 

 to the Avindward, unless it happens to be under the lee of another island. 

 On the leeward side, channels often open through to the lagoon (e, Fig. 144), 

 which, when deep enough for shipping, make the atoll a harbor ; and some 

 of these coral-girt harbors in midocean are large enough to hold all the fleets 

 of the world. 



Fig. 146 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 150 feet), the bottom is covered here and there with 

 growing corals. Between a and b there is a platform of coral rock, mostly 

 bare at low tide, but covered at high, having a width usually of about a 



146. 



0^!i,™„„. 



hundred yards : there are shallow pools in many parts of it, abounding in 

 living corals 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 oOO or 400 

 yards". At c is the beach on the lagoon side, and the commencement 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 150 feet there are no growing corals, except some 

 kinds that enter but sparingly into the structure of reefs. 



2. 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. In 

 some parts it contains imbedded corals ; in others, it is as compact as any 

 Silurian limestone and without a fossil of any kind, unless an occasional 

 shell. 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 made of corals standing as they grew, in 

 sheltered waters, where the sea has free access. Large portions are a coral 

 and shell conglomerate. 



3. Coral beach-rock. — The beach-rock is made from the loose coral sands of 

 the shores, which are thrown up by the waves and winds. The sands become 



