TAHITI. 
315 
In other words, we may regard this part of Tahiti as a 
submerged mountain 3,600 feet high. Up to a contour 
line of 3,000 feet the ground shelves upwards, at first 
gently, then more steeply, till it attains a slope of 30° ; 
from the top of this rises a line of cliffs about 350 feet high 
crowned by a slope of which the angle is 18°. The craggy 
zone is strewn, we are told, with large masses of coral — 
like a talus beneath a line of cliff — mingled with fine d&bris ; 
in about 100 yards there is only sand, which continues to 
the lower part of the mountain, where it gives place to mud, 
composed of volcanic and coral sand and various organisms, 
generally minute. 
Great stress is laid on the occurrence of this area of 
coral crags and ‘ screes ’ as indicative of the mode in which 
a reef is enabled to grow outwards on a foundation, built 
from its own ruins. Such a mode of enlargement (as Pro- 
fessor Dana points out) had, however, been obviously ad- 
mitted as possible in particular cases, 1 and so cannot be 
regarded as contrary to the general hypothesis put forward 
by Mr. Darwin. But he calls attention to the fact that the 
above observations prove : (1) that the currents round Tahiti 
are evidently weak because they carry little coral debris so 
far as a mile from the edge of the reef ; (2) that very large 
masses of coral are lying about below the submarine cliffs 
at depths of from 240 to 600 feet, i.e. far below the depth 
at which the waves could exert any serious rending force. 
The position of these blocks, always below 240 feet— too 
far from the edge of the reef to have been borne from it 
and washed at last over the brow of the steep declivity — 
seems only to be explicable when it is regarded as indica- 
ting a stage in the past history of the reef, and is a me- 
morial of a time when this declivity was the edge of a 
growing reef, and its brow was beaten by the waves. 2 
1 See pp. 22, 67, &c. of the present volume. 
2 Professor Dana considers that waves do little rending below the 
