388 NOTICES OF NEW BOOKS. 



On examining the map with a view to determine the value of 

 the theory of subsidence, it is at once seen, that while atolls and 

 barrier reefs are generally near together, the fringing reefs are 

 either distant from these, or in the exceptional cases, there is in- 

 dependent evidence of oscillations of level have taken place, sub- 

 subsidence having preceded the elevation of the latter, and eleva- 

 tion having preceded the subsidence of the former class. 



There is however some direct evidence in the external appear- 

 ance, of many of the atolls that subsidence has taken place ; 

 fissures formed by earthquakes and other recorded phenomen 

 sufficiently proving that subterraneous movement of this kind is 

 in progress, while, on the other hand, with regard to the fringing 

 reefs, they are all situated in districts where elevation of the sur- 

 face would seem to be going on. The Society Isles, and the 

 numerous islands and reefs of the Low Archipelago, are remark 

 able instances of subsidence, while the Sandwich Islands and the 

 Philippines are equally striking examples of recent elevation. It 

 is worthy of notice also, that in the vicinity of those areas of sub- 

 sidence which are marked by the presence of coral reefs of the 

 barrier and atoll kind, there is a total absence of volcanoes of recent 

 action, while they abound in the districts where ringing reefs occur. 



In conclusion, Mr. Darwin directs attention to the vast extent 

 of the areas of subsidence and elevation on the earth's surface, 

 whether as indicated by the existence of coral reefs, or by the plain 

 evidence of observed appearances. It seems that in the space of 

 ocean extending from the southern end of the Low Archipelago to 

 the northern end of Marshall Archipelago, a length of 4500 miles, 

 every island with one exception is atoll-formed. With regard to 

 the district we are told : — 



" The eastern and western boundaries (the continents of Africa and S. 

 America) are rising areas, the central spaces of the great Indian and Pacific 

 oceans are mostly subsiding ; between them, north of Australia, lies the most 

 broken land on the globe, and there the rising parts are surrounded and pene- 

 trated by areas of subsidence, so that the prevailing movements now in pro- 

 gress seem to accord with the actual states of surface of the great divisions of 

 the world" Page 143. 



Since, therefore, it appears, that while the surface of different 

 kinds of coral reefs differs but little, the barrier reef or ring of 

 coral surrounding an island becoming an atoll when the land dis- 

 appears, and being distinguished from the fringing reef by the pre- 

 sence of a deep water lagoon or moat between the reef and the 

 central expanse ; and since also the coral animal is found not to 

 exist except at very limited depths compared with the actual depth 

 of coral in the reefs, it results that — 



" There is no difficulty respecting the foundations on which fringing reefs 

 are based, while with barrier reef and atolls there is a great apparent difficulty 

 on this head ; — in barrier reefs, from the improbability of the rock of the coast 

 or of banks of sediment extending in every instance so far seaward within the 

 required depth ; and in atolls, from the immensity of the spaces over which 

 they are interspersed, and the apparent necessity for believing that they are all 

 supported on mountain summits, which, although rising very near to the sur- 



