Coral Reefs and Islands. 277 



A vertical surface below the sea-level of 20 feet made bare 

 for the broadside stroke is probably very rarely exceeded even 

 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 7 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 subsidence 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 r 

 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 

 resumed 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 consequent reef- widening in the course of a pro- 

 gressing 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 different 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 

 * Silliman's American Journal [2] xxxv. p. 197. 



