182 J. D. Dana — Origin of Coral Reefs and Islands. 



make few equal 2, meaDs 1200 feet or more, which leaves a 

 long interval yet unfilled.* 



It is also urged that some of the " emerged volcanic moun- 

 tains situated in the ocean basins " may have been wholly swept 

 away and left with a few fathoms of water above them. But 

 this is claiming more from the agents of erosion than they could 

 possibly have accomplished, as the existence of an atoll in the 

 ocean and the examples on coasts of wave and tidal action 

 prove. 



D. To give completeness to the hypothesis which makes 

 barrier and atoll islands out of sub-marine banks (whether these 

 banks have a basis of volcanic or other rocks, or of calcareous 

 accumulations), it is necessary to show that the waters of the 

 waves and currents can make barrier islands and atolls out of 

 such banks without subsidence; and explanations to this 

 effect have been given. 



It is urged, in agreement with Darwin, that the outer por- 

 tions of reefs increase faster than the inner, owing to the purer 

 water about them and the more abundant life for food ; that 

 the inner parts are not only at a disadvantage in these respects 

 but suffer also from coral debris thrown over them. They add 

 to these causes of unequal growth mentioned by Darwin, the 

 solvent and abrading action of the waters. 



It is hence concluded that, under these conditions, the sim- 

 ple bank of growing corals may have a depression made at 

 center, which, as the process continues, will become a lagoon 

 basin, and the reef, thereby, an atoll with a shallow lagoon ; 

 that the atoll, so begun, may continue to enlarge through the 

 external widening of the reef and the further action of current- 

 abrasion and solution within ; or, in the case of fringing reefs, 

 that the change may go on until the reef has become a barrier- 

 reef with an inner channel and inner reefs. It is admitted that 

 subsidence may possibly have helped in the case of the deepest 

 lagoons. 



Dr. Geikie expresses his opinion on the subject thus : "As 

 the atoll increases in size the lagoon becomes proportionally 

 larger, partly from its waters being less, supplied with pelagic 



* The actual depths over the elevations in the Tuscarora section between the 

 Hawaian Islands and Japan, numbering them from east to west, are as follows : 

 1, 11,500 feet; 2, 7500 feet; 3, 8400 feet; 4, 12,000 feet; 5, 9000 feet (this 

 seven miles west of Marcus Island) ; 6, 9600 feet. "Whether ridges or peaks the 

 facts do not decide; probably the former. No. 1 has a base of 185 miles with the 

 mean eastward slope 40 feet per mile (=1 :132) and the westward 128 feet per 

 mile. No. 2 has a breadth of 396 miles', with the mean eastern slope mostly 37 

 feet per mile, but 51 feet toward the top, and the westward, 55 feet per mile 

 (=1:96). No. 3 was the narrowest and steepest, it being about 100 miles broad 

 at base and having the mean eastern slope 192 feet per mile and the mean west- 

 ern 200 feet. 



