RELIEF FEATURES 5 



off the continental shelves. If the bottoms of the ocean basins were 

 to sink 600 feet, or a little more, all the water would be drawn off 

 the continental shelves, and they would then become land. If the 

 continents were to sink, the waters of the sea would spread over 



Fig. 2 . Land and water hemispheres. 



their low borders, and the area of the land would be lessened. The 

 history of the earth teaches that the areas of the ocean and land 

 have changed somewhat from time to time. The lower parts of the 

 continents have been drowned repeatedly, but it is not known that 

 any part of the deep sea bottom was ever land, or that any part of 

 the land was ever beneath a deep sea. 



RELIEF FEATURES OF THE SECOND ORDER 



The more strongly marked features of the continents and of the 

 ocean basins are relief features of the second order. The continental 

 areas are made up of plains, plateaus, and mountains. Some of their 

 relations are shown in Fig. 1. 



Plains 



Plains are the lowlands of the earth, yet they cannot be defined 

 in terms of height above the sea. They m&y be but a few feet above 

 it, or they may be hundreds or even thousands of feet above; but if 

 so high as a thousand feet, they are generally far from the sea, and 

 distinctly lower than some of the other lands about them. Plains 

 differ widely among themselves ; not only in height, but in position, 

 in size, in shape of surface, in fertility, in origin, and in various other 



