DEPOSITION IN THE SEA 255 
To explain the latter it has been supposed that the atoll began 
as a fringing reef surrounding a volcano, or other mountain peak, 
which was raised above the surface of the ocean (Fig. 145). This 
peak was slowly sinking, and as it sank the coral grew upward, 
forming first a barrier reef, then, when the peak entirely disap- 
peared, an atoll (Fig. 144). 
Fig. 145. 
A voleanic island in the Pacific, surrounded by a coral reef. The submergence 
of the island might leave an atoll. 
This theory was suggested by Charles Darwin and advocated 
by Professor Dana; but to many it has seemed improbable, for 
there is such a large number of atolls and other coral reefs in the 
Pacific, that if the theory is true, it must be believed that there is 
a great subsidence in the bottom of the Pacific Ocean in progress 
Over animmense area. This subsidence must be going on at a very 
