ORIGIN OF BARRIER REEFS AND ATOLLS. 273 
islands or their barriers, show no tendency to grow with 
large depressed centres, but rather with flat. tops, as vegeta- 
tion might grow, or else with elevated centres. It is only 
when nearing the surface where the waves can help vigor- 
ously the growth and the accumulation of material on the 
border and injure the interior corals, that anything like 
a lagoon-basin begins and the atoll takes shape; and it is 
only through continued subsidence under such conditions 
that the margin can be made to grow so much faster than | 
the interior as to produce thereby a basin-like interior 50 to 
300 feet deep. Corals will grow most rapidly where food is 
most freely supplied by currents, as observed by Mr. Agassiz. 
The principle serves to explain the unequal progress in some 
reef-banks; but food is seldom deficient. Darwin draws a 
good argument for his theory, also, from the fact that lines 
of coral islands are the continuations of lines of high islands, 
and, also, that lines of the two kinds of islands often run 
parallel with one another; for example, the Hawaiian 
line of high islands four hundred miles long, ending off to 
the westward in a longer line of atolls, and the many 
parallel lines in the Pacific. | 
There is, further, not merely probable but positive evi- 
dence of subsidence in the deep coast-indentations of the 
high islands within the great barriers. The long points and 
deep fiord-like bays are such as exist only where a land, after 
having been deeply gouged by erosion, has become half sub- 
merged. The author was led to appreciate this evidence 
when on the ascent of Mt. Aorai on Tahiti, in September of 
1839." Sunk to any level above that of five hundred feet, 
1 A map of Tahiti and an account of the ascent are contained in the author’s 
“Volcanoes and Hawaiian Volcanic Phenomena.” ‘The map is published, also, in 
the American Journal of Science, 1886, xxxii, 247. 
18 
