244 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



situated on the border of a submarine plateau or ledge. Such a position, according 

 to the explanation of barrier-reefs, first advanced by LeConte, and supported by 

 myself, presents the most favorable conditions for reef growth, the corals being limited 

 on the outside by the depth, and on the inside by the sediment in the water. The 

 influences of food-supply and currents act subsequently as auxiliary causes. 



What, then, is the explanation of the submarine ledge? The supposition that it is 

 a continuation of the land slope is at once negatived by the fact that the slope of the 

 land in the reef -encircled islands of the Pacific is usually 6 degrees or 7 degrees, some- 

 times only 3 degrees or 4 degrees, but often as much as 10 degrees, or 12 degrees, whilst 

 the submarine ledge, when stripped of reefs and defined by the 100-fathom line, would 

 possess a scarcely recognizable inclination, represented by a fraction of a degree. 

 It will be found, however, when we examine the contour of such an island as Vanikoro, 

 that the distance of the barrier-reef from the coast may vary according to the slope of 

 the land. Thus, on the west side of this island, the average angle of the land slope is 

 6 degrees, and the distance of the barrier reef about 2^ miles. On the north side the 

 inclination of the land is between 11 degrees and 12 degrees, and the barrier reef is 

 rather over a mile distant. This is just what we should expect. The more gradual the 

 land slope, the broader will be the submarine ledge, cut out in the course of ages by 

 the action of the sea, and the more distant will be the barrier reef that has grown up 

 along its margin. This I believe to be the true explanation of the position of barrier 

 reefs. A submarine ledge is in the first place necessary; and, since the sediment and 

 mud in the shallower waters on the ledge repress the growth of corals, reefs will 

 naturally spring up toward the margin of the ledge, where the water is clearer and 

 where the depth is within that of the reef-coral zone. 1 



5. Admiral Sir W. J. L. Wharton 2 explained the uniform depth of 

 atoll lagoons, whose edges are in various degress encircled by growing 

 coral, by considering that the corals grow upon foundations that are 

 the bases of volcanic islands that have been reduced by wave action 

 to wave base. 



6. Alexander Agassiz 3 found older limestone under the recent reefs 

 in many areas investigated by him. He explained atolls by the solu- 

 tion and erosion of the interior of preexisting limestone masses and 

 ascribed the formation of the platforms of barrier reefs to marine 

 erosion without change of sea level. 



7. Andrews 4 pointed out that the platform of the Great Barrier 

 Reef of Australia has been submerged at a relatively recent date and 

 that it continues southward beyond the reef, and he inferred that only 

 a minor part of the platform is "formed of coral growth." 



8. The opinions of Stanley Gardiner 5 are closely in accord with those 

 of Semper, Murray, Wharton, and Agassiz. According to him sub- 

 marine planation is effective to depths as great as 200 fathoms. 



1 Guppy, H. B., The origin of coral reefs, pp. 60, 61. 



2 Wharton, W. J. L., Foundations of coral atolls, Nature, vol. 55, pp. 390-393, 1897. 



3 Agassiz, Alexander, The Coral reefs of the Tropical Pacific, Mem. Mus. Comp. Zool., vol. 27, 1 vol. of 

 text, 3 vols . of pis . , 1903 . 



4 Andrews, E. C, Preliminary note on the geology of the Queensland coast with references [to the 

 geography of the Queensland and N. S. Wales Plateau, Proc. Linn. Soc. New South Wales, pt. 2, pp. 146- 

 185, 1902. 



6 Gardiner, J. Stanley, The formations of the Maldives, Geographical Journal, pp. 277-296, March, 1902; 

 Fauna and geography of the Maldive and Laccadive Archipelagoes, pp. 182, 183, 1901-3. 



