366 CORALS AND CORAL ISLANDS. 



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 Trop- 

 ical 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. 327) 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 author has presented reasons for believing [Am. J. Sci., 

 III. ii. 1871) that in this Glacial era the watershed of Canada, 

 between the Piver St. Lawrence and Hudson's Bay, was 

 raised at least 5,500 feet above its present level (1,500 feet); 

 and that this plateau thus elevated was the origin of the great 

 glacier which moved southeastward over New England. This 

 region is the summit of the eastern arm of the great V-shaped 

 Azoic area of the continent, the earliest elevated land of North 

 America ; and it is not improbable that the other arm of the 

 V, reaching from Lake Superior and Huron, northwestward, 

 to the Arctic, was raised at the same time to a higher elevation, 

 and was the source of glacial movements over the more central 



