372 
description above of a typical atoll that such a reef 
extends outwards on its own talus to form a great 
broad plateau. Boring organisms enter on its central 
part and cause the rock to decay. Sand-feeders follow 
SEA 
LEVEL 
ill sf 
ol 
voc 
Fic. 1.—Diagram to illustrate the first method of formation of an atoll. 
A, Original mvund on the sea floor. B 1-4, Growth of the same to furm 
a surface reef. C 1-3, Extension outwards and formation of an atoll. 
and triturate up the fragments, throwing a constant 
stream of fine mud into suspension in the w ater to be | 
removed by the tidal and other currents. Assisted by 
the solubility of coral in sea-water, a lagoon is formed 
in the centre of the reef, and passages are cut later 
from it along the lines by which its muddy water 
escapes. The process may continue to form an atoll 
of the largest size, such even as Funafuti or any of | 
the Ellice and Gilbert groups, which appear to have | 
sae on a single mountain range. | 
(2) (Figs. 2 and 3) The second method of formation | 
depends on the power of the ocean currents to cut 
down land and form submarine banks. It is exempli- 
fied by the Maldive Group, the main chain of which 
is more than 300 miles long, and lies at right angles 
to the monsoon currents oi the Indian Ocean. Here 
the action of the currents appears to have cut down | 
a great tract of land, or at least a series of peaks, to 
form a plateau more than 100 fathoms in depth (Fig. 
3). It is easy to see how the loose mass of cinders 
formed in a submarine eruption might be so cut down | 
to 30 or even 50 fathoms, but this action, when first 
proposed for the Maldives, seemed extreme. It has, | 
however, received strong support from the work of the | 
Siboga Expedition in the East Indies. At depths | 
below 50 fathoms it is obvious that from the first the 
organisms on the periphery of the bank so formed | 
w auld grow up more rapidly, and so an atoll as such | 
would directly arise (Fig. 2). The whole action might 
proceed extremely rapidly. Indeed, it is not unlikely 
that the shoal marking the site of what was once 
Sea Lever 
Fic. 2.,—Diagram of the second method of formation of an atoll. 
of the orginal island, ., 
c, The atoll reef. 
A, Contour 
The same cut down to form a plateau 
Falcon Island, Tonga, will by 4ooo a.p. be occupied | 
by a considerable atoll. The action on a land like the 
Maldives, which is of considerable linear extent and of 
more solid construction, would proceed more slowly | 
and show many modifications. A certain number otf | 
NO. 1790, VOL. 69] 
NATURE 
[ FEBRUARY 18, 1904 
channels would be retained and even deepened as the 
rim grew up. Each piece of rim as it formed would 
give a protected area behind itself around which the 
currents would sweep, and might thus become separ- 
ated as a distinct bank or atoll. The same action 
might occur at two or three levels, and in this way 
the whole bank, instead of being crowned by one atoll 
ring, might be surrounded by a series of secondary 
atolls, their rims again formed by series of still smaller 
tertiary rings. This, in truth, appears to be the case in 
the Maldives, but the continued upgrowth of the rims 
of the secondary atolls is uniting the outer sides of the 
700 
100 
200 
Hl if 
4 
RET AE RTL 
PBS HIF HEE a a 
Fic. 3.—Section of th outer edge of one of the Maldive atolls. a, Founda- 
tion of primitive rock cut down by the currents. BB’, Upgrowth of th 
rim by the deep-sea, intermediate depth and reef organi-ms. c, Exten- 
sion outwards by means of the talus slope. bp, Lagoon. (Scale in 
Yathoms.) 
tertiary rings, while their inner parts are being re- 
moved. 
(3) (Fig. 4) A flat terrace is formed around the shore 
of an island by the action of the sea on the land, and 
is covered at its edge by reef organisms, or a fringing 
reef is formed. Subsequently, the edge of the terrace 
grows outwards and its inner part is removed as in 
(1), formine a barrier reef. Eventually, the original 
island, owing to similar causes, disappears, leaving 
an atoll. This method is of quite wide occurrence in 
Fic. 4.—Sections across (1) Ongea L2vu, Fiji, limestone, and (2) Wakaya, 
Fiji, volcanic, to illustrate the third method of formation. a, Supposed 
contour of the ori. inal land. 88, Section across the existing land and 
reef. (Vertical scale many times the horizontal. 
areas where elevated coral reefs are found (Fig. 4, 1). 
Every stage can be followed in the limestone islands 
of the Fiji group. It also occurs, though much more 
slowly, around other islands. A good example is 
Wakaya, Fiji, which has to the west a broad fringing 
reef, half formed by a terrace cut out of the land and 
half by a true coral formation, and to the east a barrier 
reef separated by a channel 33 miles from the land, 
which still retains a definite terrace of the volcanic 
rock (Fig. 4, 2). 
