vi.] ON CORAL AND CORAL REEFS. 129 



or is undergoing a slow upheaval, in the neighbourhood 

 of active volcanoes ; and, therefore, neither atolls nor 

 encircling reefs ought to be found in regions in which 

 volcanoes are numerous and active. And this turns out 

 to be the case. Appended to Mr. Darwin's great work 

 on coral reefs, there is a map on which atolls and en- 

 circling reefs are indicated by one colour, fringing reefs 

 by another, and active volcanoes by a third. And it is 

 at once obvious that the lines of active volcanoes lie 

 around the margins of the areas occupied by the atolls 

 and the encircling reefs. It is exactly as if the up- 

 heaving volcanic agencies had lifted up the edges of 

 these great areas, while their centres had undergone a 

 corresponding depression. An atoll area may, in short, 

 be pictured as a kind of basin, the margins of which 

 have been pushed up by the subterranean forces, to 

 which the craters of the volcanoes have, at intervals, 

 given vent. 



Thus we must imagine the area of the Pacific now 

 covered by the Polynesian Archipelago, as having been, 

 at some former time, occupied by large islands, or, may 

 be, by a great continent, with the ordinarily diversified 

 surface of plain, and hill, and mountain chain. The 

 shores of this great land were doubtless fringed by coral 

 reefs ; and, as it slowly underwent depression, the hilly 

 regions, converted into islands, became, at first, sur- 

 rounded by fringing reefs, and then, as depression went 

 on, these became converted into encircling reefs, and 

 these, finally, into atolls, until a maze of reefs and 

 coral-girdled islets took the place of the original land 

 masses. 



Thus the atolls and the encircling reefs furnish us 

 with clear, though indirect, evidence of changes in the 

 physical geography of large parts of the earth's surface ; 

 and even, as my lamented friend, the late Professcr 



H K 



