THE OCHANIC CORAL-ISLAND SUBSIDENCE. 4(1)3 
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 adownward 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. 357) 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 eleva- 
tion 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 beleving (Am. J. Sci., 
III. v. 1873) that in this Glacial era the watershed of Canada, 
between the River St. Lawrence and Hudson’s Bay, was 
raised somewhat 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 
Archean 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, northwest- 
ward, to the Arctic, was the source of glacial movements over 
