396 PRINCIPLES OF STRATIGRAPHY 



the debris of the reef organisms with i)lentiful HaHmeda sands, but 

 the windward islets are largely composed of the material torn from 

 the reef platform, and sand is scarce. 



In places where the islets are absent, the leeward rim passes 

 gradually from an area covered by living organisms nearest the 

 ocean into a central portion of perfectly bare and smooth Litho- 

 thamnion rock. Thence to the edge next to the lagoon the condi- 

 tions of the ocean face of the reef repeat themselves, but on a 

 smaller scale. 



On the lagoonward slopes sands abound, consisting either wholly 

 of Halimeda fragments or of these mixed with Foraminifera and 

 shell fragments. On the seaward side the platform generally ends 

 in a cliff or vertical wall, one or two fathoms deep, beyond which 

 the general submarine slope of the atoll begins. 



Structure of Coral Reefs. 



In most modern coral reefs it is found that the lens-shaped clus- 

 ters of growing corals or reef mounds are scattered over the foun- 

 dation platform "like tufts of vegetation in a sandy plain" (Dana- 

 20: 174), and that tliey are surrounded and connected by coral sand 

 resulting from the destruction of the reefs by the waves and by 

 organisms. The individual reef mound generally rises as an iso- 

 lated lens-like mass above the sea bottom to near the surface of the 

 water, and is composed of coral heads and branching forms, between 

 which coral sand and shells and shell fragments of molluscs, echino- 

 derms and other organisms accumulate. The form varies from a 

 shallow lens, which is almost a layer, to that with lateral slopes as 

 high as 45 degrees or over. A peculiar variety known as "cha- 

 peiros" were described by Professor Hartt (46: ipi, ipp-200) from 

 the coast of Brazil. These grow in small, scattered patches, "and, 

 without spreading much, often rise to the height of forty to fifty 

 feet or more, like towers, and sometimes attain the level of low wa- 

 ter ... at the top they are usually very irregular, and some- 

 times spread like mushrooms or . . . umbrellas. Some of these 

 chapeiros are only a few feet in diameter." They grow thickly 

 scattered over considerable areas in those waters. According to 

 Dana, "the rock of these submerged coral heads is but a loose ag- 

 gregation of coral in the position of growth, except possibly in their 

 lower portion, where the open spaces may be filled with sand and 

 fragments cemented together." 



Except where reefs rise from great oceanic depths, their sub- 



