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



Y. — Other methods of explanation, and their supporting evidence. 



A. Mr. John Murray, one of the able naturalists of the Chal- 

 lenger Expedition, reports the following important results from 

 soundings off northern Tahiti, made under his supervision and 

 that of the surveying officer.* 



Along a line outward from the edge of the barrier reef there 

 were found : (1) for about 250 yards, a shallow region covered 

 partly with growing corals, which deepened seaward to 40 fath- 

 oms ; (2) for 100 yards, between the depths of 40 and 100 fathoms, 

 a steeply but irregularly sloping surface which commenced with 

 a precipice of 75° and had a mean angle exceeding 45° ;f then 

 (3) for 150 yards a sloping bottom 30° in angle ; (4) then a con- 

 tinuation of this sloping surface, diminishing in a mile to 6°, at 

 which distance out the depth found was 590 fathoms (3,540 

 feet). Over the area (2), or the 100 yards between 40 and 100 

 fathoms, the bottom was proved to be made of large coral 

 masses, some of them "20 to 30 feet in length," along with 

 finer debris ; outside of this, of sand to where the slope was re- 

 duced to 6° ; and then of mud, composed " of volcanic and 

 coral sand, pteropods, pelagic and other foraminifers, coccoliths, 

 etc." 



These observations have great significance. They show (1) 

 that the feeble currents off this part of Tahiti carry little of the 

 coral debris in that direction beyond a mile outside of the grow- 

 ing reef ; (2) that a region of large masses of coral rock and 

 finer material occurs at depths between 240 and 600 feet ; (3) 

 that, a mile out, the bottom has the slope nearly of the adjoin- 

 ing land, and in this part is covered with the remains of pelagic 

 life. 



From the second of these facts — the great accumulation of 

 coral blocks below a level of 240 feet — Mr. Murray draws the 

 conclusion that, in the making of fringing, barrier and atoll 

 reefs, the widening goes forward (a) by making first upon the 

 submarine slopes outside of the growing reef a pile of coral 

 debris up to the lower limit of living reef-corals; and then (b) 

 by building outward upon this accumulation as a base. 



He also announces, after speaking of other causes influencing 

 the growth of corals, the more general conclusion that "it is not 

 necessary to call in subsidence to explain any of the character- 

 istic features of barrier reefs and atolls," and concludes that his 

 views "do away with the great and general subsidences" ap- 

 pealed to by Darwin. 



a. The widening-process, in the first conclusion, had previ- 



* Proc. Edinburgh Roy. Soc, Session 1879-80, p. 505. 



f Dr. Geikie gives in his paper a section of the soundings, " on a true scale, 

 vertical and horizontal," and in it the upper steepest part of this 100 yards has a 

 slope of about 75°. 



