176 J. D. Dana — Origin of Coral Reefs\and Islands. 



ported cases of like steepness at greater depths ; for example, 

 the sounding of Captain Fitzroy at Keeling atoll (while Darwin 

 was there), 2200 yards from the breakers, in which no bottom 

 was found at a depth of 1200 fathoms, but the line was partly 

 cut at a depth between 500 to 600 fathoms; the sounding by 

 the Wilkes Expedition off Clermont Tonnerre(Paumotu Archi- 

 pelago), where the lead brought up an instant at 350 fathoms, 

 and then dropped off again, descended to 600 fathoms without 

 reaching bottom, and came up bruised, with small pieces of 

 white and red coral attached; a sounding by the same Expedi- 

 tion, a "cable's length " from Ahii, in which the lead struck a 

 ledge of rock at 150 fathoms and brought up finally at 300 

 fathoms.* All the older soundings need to be repeated ; but 

 there must be enough truth in those quoted to warrant the 

 remark that the force of Darwin's argument for subsidence from 

 the steepness of the submarine slopes about atolls is not weak- 

 ened by the Challenger results. 



d. But the chief interest of the Challenger soundings con- 

 sists in their affording "direct" proof, "positive" proof, of 

 much subsidence ; a kind of proof that subsidence sinks out of 

 sight, and which soundings may yet make available in many 

 similar cases. 



That belt of coarse debris — including " masses 20 to 30 feet" 

 long — was found over the steeply sloping bottom at depths 

 between 240 and 600 feet. These depths are far below the 

 limit of forcible wave-action. They are depths where the 

 waters, however disturbed above by storms, have no rending 

 and lifting power, even when the bottom is gradually shelv- 

 ing ; depths, in this special case, against a slope which for 100 

 yards is 75° in its upper part, and in no part under 45°, the 

 vertical fall being 360 feet in the 100 yards. Strokes against 

 the reef-rock thus submerged, and under such conditions, would 

 be extremely feeble. Waves advancing up a coast, whether 

 storm-driven waves or earthquake-waves, do little rock-rend- 

 ing below the depth to which they can bare the bottom for a 

 broadside plunge against the obstacle before them, although 

 the velocity gives them transporting power to a greater depth. 

 It is the throw of an immense mass of water against the front, 

 with the velocity increased by the tidal flow over a shelving 

 bottom, — the rate sometimes amounting, according to Steven- 

 son, to 36 miles an hour or 52'8 feet a second, — together with 

 the buoyant action of the water, that produces the great 

 effects. 



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

 for the broadside stroke is probably very rarely exceeded even 



* Ibid., page 55. 



