OCEANIC CORAL ISLAND SUBSIDENCE. 315 



reckoning only from the Sandwich Islands to the Friendly Group 

 (or to Tongatabu) is over twenty-five hundred miles, thus 

 equalling the width of the North American continent. A 

 movement of such extent, involving so large a part of the earth's 

 crust, could not have been a local change of level, but one in 

 which the whole sphere was concerned as a unit ; for all parts, 

 whether participating or not, must have in some way been in 

 sympathy with it 



This subsidence was in progress, in all probability, during 

 the Glacial era, the thickness of the reefs proving that in their 

 origin they run back through a very long age, if not also into 

 the Tertiary. It was a downward movement for the tropical 

 Pacific, and perhaps for the warmer latitudes of all the oceanic 

 areas, while the more northern continental lands, or at least 

 those of North America, were making their upward movement, 

 preparatory to, or during that era of ice. 



The subsidence connected with the origin of coral islands 

 and barrier reefs in the Pacific has been shown (p. 281) to have 

 amounted to several thousands of feet, perhaps full ten thou- 

 sand. And it may be here repeated, that, although this sounds 

 large, the change of level is not greater than the elevation 

 which the Rocky Mountains, Andes, Alps, and Himalayas 

 have each experienced since the close of the Cretaceous era, 

 or the early Tertiary ; and perhaps it does not exceed the 

 upward bulging in the Glacial era of part of northern North 

 America.^ 



The northern continental upward movements which intro- 

 duced the Glacial era, carrying the Arctic far toward the 

 Tropics, may have been a balance to the downward oceanic 

 movements that resulted in the formation of the Padfic 

 atolls. While the crust was arching upward over the former 

 (not rising into mountains, but simply arching upward) it may 



^ The arguments which have seemed to favour the view that the por- 

 tions of North America in the higher latitudes, and probably also in the 

 corresponding parts of the other continents, were above I heir present 

 level, are briefly presenttd by the author in his " Manual of Geology," 

 and also, recently, in the Amerkan Journal of Science, third series, 

 volume V. 



