STRUCTURE OF CORAL REEF 399 



where they form an effective protection against the breakers. They 

 form an artificial breakwater rising about 3 feet higher than the 

 rest of the reef. This is a very common occurrence in modern 

 reefs. Steep slopes are found around many modern atolls, and it 

 appears that where the currents run with greatest force near the 

 reef the slopes are steeper. In the Alaldive and Chagos atolls such 

 steep slopes are common. At Heawandoo Pholo 50 and 60 fathoms 

 were found close to the edge of the reef, and at 300 yards' dis- 

 tance no bottom was obtained with a 300-yard line. On Egriiont 

 Island a slope of 45° was found, while the slopes on the Cardoo 

 atoll were so great that no bottom was obtained with a line 200 

 fathoms long at only sixty yards from the reef. 



The general character of the slopes on the Low Archipelago 

 closely corresponds to that found on the Keeling atoll, the slope a 

 short distance beyond the edge of the reef being 45°. In some 

 cases, however, the slope is that of a vertical cliff. Perpendicular 

 or even overhanging cliffs are, however, also found in the quiet 

 water of the lagoon, while in other cases, where the water is gen- 

 erally tranquil, though not always so, as on the leeward side of 

 IMauritius, the slope is a very gentle one. 



The shores of the lagoons within the atolls also vary greatly. 

 At Keeling atoll (Fig. 78) they shelve gradually where the bottom 

 is of sediment, and irregularly or abruptly where there are coral 

 reefs. In the Marshall group, on the other hand, the slope is often 

 abrupt so that we pass directly "from a depth of two or three 

 fathoms to twenty or twenty-four, and you may pursue a line in 

 which on one side of the boat you may see bottom, and on the other 

 the azure blue deep water." (Chamisso, quoted by Darwin-21.) 

 In the Matilda atoll of the Low Archipelago the surface of the great 

 exterior reef slopes gently toward and beneath the surface of the 

 lagoon, and then ends abruptly in a little cliff 3 fathoms deep. At 

 its foot a ledge forty yards wide extends, shelving gently inward 

 like the surface reef, and is terminated by a second little cliff 5 

 fathoms deep, beyond which the bottom of the lagoon slopes off 

 to twenty fathoms, its average central depth. The ledges appear 

 to be formed of coral rock, often porous, so that the sounding lead 

 descends several fathoms through holes in them. Some lagoons, 

 on the other hand, have gentle slopes to the center. 



Sections made by Captain Maclear, R. N., of the small island 

 of Masamarhu in the Red Sea show not only the nature of the 

 slopes of a coral island, but also give indisputable proof of sub- 

 sidence since the commencement of the growth. "In each case the 

 surface of the fringing reef, after shelving very gently downward 



