STRUCTURE OF CORAL REEF 



397 



merged slopes are generally low, frequently falling much below lo 

 degrees. This is particularly the case in the Bermudas, where the 

 slopes are very gentle. The following are given for Hamilton 

 Island in this group. (Dietrich, quoted by Walther-90 : poj.) 

 Westward the shallow reef extends for 19.4 kilometers, then for the 

 next 8.9 kilometers the slope averages 21 degrees 50 minutes, carry- 

 ing the depth to 3,566 meters. Then for the next 11 kilometers it 

 rises again at an average angle of one degree 20 minutes to a 

 depth of 3.310 meters, falling again for the next 36 kilometers at an 

 average angle of 2 degrees T.y minutes to 4,846 meters in depth. On 

 the same island the slope to the south-southwest, after passing the 

 reef which extends for 4.3 kilometers, averages 3 degrees 2,2 min- 



FiG. 78. Section of Keeling atoll, drawn from the outer coast at low water, 

 across one of the small islands to the lagoon. A — Level of the sea 

 at low water. At A the depth is 25 fathoms, and the distance 

 rather more than 150 yards from the edge of the reef. B — Outer 

 edge of flat part of reef, which dries at low water ; the edge either 

 consists of a convex mound, as represented, or of rugged points, 

 like those a little farther seaward, beneath the water. C — A flat 

 coral rock covered at high tide. D — A low projecting ledge of 

 brecciated coral rock washed by the waves at high water. E — A 

 slope of loose fragments, reached by the sea only during gales ; 

 the upper part, which is from six to twelve feet high, is clothed 

 with vegetation. The surface of the islet gently slopes to the 

 lagoon. F — Level of the lagoon at low water. (After Darwin.) 



utes for a distance of 20.5 kilometers, carrying the depth to 1,234 

 meters. Then it rises again at an angle of 3° 8' for the next 5.5 

 kilometers until the depth is only 933 m. Following this a slope of 

 3° 55' for 8 km. carries the depth again to 1,481 meters, beyond 

 which for the next 26.7 km. the average angle increases to 4° 9', 

 carrying the depth to 3,420 meters. Southward after passing the 

 reef (2-9 km. from shore), the margin of the reef falls at an angle 

 of 8° 11' to a depth of 3,328 meters, which is reached 23.1 km. 

 from the edge of the reef. On the north side of David Island the 

 reef extends for 15.35 l^"i-> then falls at an angle of 10° 8' to a 

 depth of 2,798 meters, reached at a distance of 15.65 km. from the 

 edge of the reef. On the Bermudas the angle of slope of the edge 

 of the reef varies from 3° 22' toward the S.S.W. to 8° 11' toward 

 the S., 10° 8' to the north and 21° 50' to the west. 



