MR. DARWIN'S REEF-FORMATION THEORIES. 



79 



Illustrating Mr. Charles Darwin's Theories of the Origin of Barrier Reefs and 



OF Lagoon Islands or Atolls. 



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Fig. i.^After Darwin. 



AA, — Outer edge of the reef at the level of the sea. 



BB. — Shores of the island. 



A'A'. — Outer edge of the reef after its upward growth during a period of subsidence. 



CC. — The lagoon channel between the reef and the shores of the now encircled land. 



B'B'. — The shores of the encircled island. 



N.B. — In this and the following woodcut, the subsidence of the land could only be represented by an 

 apparent rise in the level of the sea. 



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A.^-T C V B' 



Fig. 2. — After Darwin. 



A'A . — Outer edges of the Barrier Reef at the level of the sea. The cocoaaut trees thus 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. 



The explanation of Mr. Darwin's diagrams is as follows : — 



"Let us in imagination place within a subsiding area an island surrounded by a fringing 



