134 ON CORAL AND CORAL REEFS 



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 encircling 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 upheaving 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 mar- 

 gins 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, surrounded 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 Professor 

 Jukes, has suggested, give us indications of the manner 



