468 
Van Rise—Earth Movements. 
with the continent, are a part of it, and the continental mass must 
be considered as ending where the great slope begins which de- 
cends into the abyss of the ocean. Thus defined, there is but one 
great continent, for the Americas, Greenland, Eurasia, and 
Africa are connected by the continental shoals; but for conven¬ 
ience the word continent will be used in the ordinary sense. 
The heights of continental areas average between 2,000 and 
2,500 feet above the level of the sea 1 although a number of 
extensive plateaus have altitudes from 5,000 to 12,000 feet, 
and a few mountains have altitudes between 20,000 and 30,000 
feet. The total vertical distance between the deepest parts of 
the ocean and the tops of the highest mountains is more than 
50,000 feet. But the great features of the earth are the deep- 
lying ocean plain and the low continental plain. 
Whether or not a greater or less portion of the continental 
masses chances to be a few hundred feet above or below the 
water is a comparatively small matter, but to bring any part 
of the oceanic bed to the surface would involve a vertical move¬ 
ment of 15,000 or more feet. Vertical movements which have 
placed all or nearly all parts of all continents below the ocean 
have recurred at various times in the past. Of vertical move¬ 
ments which have brought extensive areas of the deep-lying bed 
of the ocean to the surface we have scant evidence. 
The Permanence of Continents .— The continents and ocean beds 
alike are ever subject to omnipresent gravity. The continental 
masses down to the level of the bed of the ocean are plainly heavier 
than the water standing opposite them. This being so, two ex¬ 
planations have been offered for the permanent existence of the 
continents. Either the rocks composing them and underlying them 
must be strong enough to sustain their own enormous mass, upon 
an average 16,000 to 16,500 feet above the bed of the sea and only 
partly balanced by the water of the ocean, or else the rocks un¬ 
derlying the ocean must be heavier than those constituting the 
continents and their downward extensions. Geologists now 
believe that no such strength can be premised for the rocks as 
1 A summary of estimates is given in Text-book of Geology, by Sir Archi¬ 
bald Geikie: 3d ed., 1893, pp. 39-40 
