136 
THEORY OF THE FORMATION 
Cn. Y. 
unbroken lines, instead of by dots as in the former wood- 
cut, let the work of subsidence go on, and the doubly- 
pointed hill will form two small islands included within 
one annular reef. Let the island continue to subside, 
and the coral-reef will continue growing up on its own 
foundation, whilst the water gains inch by inch on the 
land, until the last and highest pinnacle is covered, and 
there remains a perfect atoll. A vertical section of this 
No. 6. 
A'A' — Outer edges of the barrier-reef at the level of the sea. The 
cocoa-nut trees represent coral-islets formed on the reef. 
CC — The lagoon-channel. 
B' B'— The shores of the island, generally formed of low alluvial land 
and of coral detritus from the lagoon-channel. 
A"A" — The outer edges of the reef, now forming an atoll. 
C' — The lagoon of the newly-formed atoll. According to the scale 
the depth of the lagoon and of the lagoon-channel is exaggerated. 
atoll is shown in the woodcut by the dotted lines ; — a 
ship is anchored in its lagoon, but islets are not supposed 
yet to have been formed on the reef. The depth of the 
lagoon and the width and slope of the reef, will depend 
on the different circumstances to which it has been 
exposed, as just stated with respect to barrier -reefs. 
Any further subsidence will produce no change in the 
