ORIGIN OF COR A I REEFS 87 



are usually in the form of very lofty cones rising steeply 

 from the depths of the ocean to the height of a few feet 

 above the level of the water. On their tops there is gener- 

 ally a shallow cup-shaped depression of a few score feet in 

 depth, bounded by the low f]at wall of the living reef of coral 

 sand which had been formed along the shore. The origin of 

 these deep atoll sea-reefs is not yet perfectly understood. 

 Mr. Darwin's explanation, long considered satisfactory but 

 now brought into doubt, was to the effect that they were 

 produced wherever a massive coral growth was formed around 

 the flanks of a slowly down-sinking island, the polyps build- 

 ing their limestone wall upward as the peak lowered into 

 the depths of the sea. Dr. John Murray, the well-known 

 oceanographer, has recently shown that these basin-shaped 

 reefs may be formed wherever shallows are produced in situa- 

 tions where they are swept over by a warm marine current. 

 The upward growth of the coral tends to form a cap upon 

 the shoal which slowly rises to near the level of low tide. 

 Attaining this altitude, the central part of the mass begins 

 to dissolve away, and the process of solution continues until 

 a basin is formed, while on the outside margin of the reef the 

 polyp communities continue to grow, and from the debris 

 which they afford under the beating action of the waves the 

 ring-like island is formed. Although there may be cases in 

 which the elaborate hypothesis of Mr. Darwin is applicable, 

 there can be no doubt that by far the greater part of the 

 atolls have developed in the manner indicated by Dr. 

 Murray. 



Yet another singular feature in the topograph)- of the 

 oceans is found in the great shelf-like shallows which usually 

 border the margins of the continents. These remarkable 



