J. D. Dana — Origin of Coral Reefs and Islands. 177 



in the case of earthquake-waves ; and with storm-waves, or 

 recorded earthquake-waves, the displacement of the water at a 

 depth of 240 feet would be at the most only a few inches. I 

 saw on atoll reefs no upthrown masses of coral rock over ten 

 feet in thickness and twenty feet in length or breadth. It is 

 therefore plainly impossible that such a belt of debris should 

 have been made at its present level, or even at a depth of 20 

 feet ; and hence the debris affords positive proof of a large subsi- 

 dence during some part of the reef-making era. 



The existence of the belt of debris may be explained as 

 follows : If the reef now at a depth of 240 feet were at the 

 sea-level as the sea-level reef, and subsidence were not in 

 progress for a period, the very steep front of the reef now 

 just below the 240-foot level might have resulted from the 

 widening that would have gone forward. And, under such 

 conditions, the action of the occasional extraordinary waves 

 might have torn off masses from the front which would have 

 tumbled down the steeply sloping surface until the belt of 

 debris had been formed. Then, with a renewal of the slow 

 subsidence, the thickening of the reef would have been re- 

 sumed and gone on to its final limit, and the rendings of the 

 great waves found lodgment at higher levels. The masses 

 now on atoll reefs must be from comparatively recent up- 

 throws. 



This direct evidence of subsidence from Tahiti renders it 

 reasonable to make subsidence in atoll-making a general truth. 

 It is nevertheless desirable that facts of the kind should be 

 multiplied. The abrupt descent in the submarine slopes of 

 reefs detected by Fitzroy at a depth below 3000 feet, and those 

 reported by the Wilkes Expedition at depths of 2100 and 900 

 feet, seem "to indicate a similar rest at the sea level and conse- 

 quent reef-widening, in the course of a progressing subsidence; 

 and proof of this may yet be found in belts of coarse coral- 

 rock debris at the foot of the precipices. Such a period of 

 rest would lead to the forming of submarine precipices in dif- 

 ferent regions contemporaneously at different depths according 

 to the rate of subsidence of the part of the subsiding area. 



B. From facts observed about the Florida reefs, Lieutenant 

 E. B. Hunt, U. S. N., announced, in 1863,* the conclusion 

 that these reefs had received their westward elongation through 

 the westward " sweep " of an eddy current to the Gulf Stream. 

 The subject, nearly twenty years afterwards, was more thor- 

 oughly investigated by Mr. Alexander Agassiz, and the same 



*This Journal, II, xxxv, 197. 



