274 Dr. J. D. Dana on the Origin of 



from soundings off northern Tahiti, made under his super- 

 vision and that of the surveying officers *. 



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 fathoms ; (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° t; then (3) for 150 yards a sloping bottom 

 30° in angle ; (4) then a continuation of this sloping surface, 

 diminishing in a mile to 6°, at which distance out the depth 

 found was 590 fathoms (3540 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 u 20 

 to 30 feet in length," along with finer debris ; outside of this, 

 of sand to where the slope was reduced to 6° ; and then of 

 mud, composed " of volcanic and coral-sand, pteropods, pelagic 

 and other foraminifers, coccoliths, &c." 



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 

 growing 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 

 adjoining 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 

 characteristic features of barrier-reefs and atolls;" and con- 

 cludes that his views " do away with the great and general 

 subsidences " appealed to by Darwin. 



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

 previously been a part of the Darwinian theory ; for, as 



* Proc. Edinburgh Boy. Soc, Session 1879-80, p. 50o. 



t 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°. 



