4 PHYSIOGRAPHY 



About the continents as we know them, there is almost every- 

 where a belt of shallow water. The sea bottom below this shallow 

 water is the continental shelf (Fig. 1), at the outer edge of which 

 there is a rather steep slope down to the ocean basins. 



Grouping of the continents. The northern hemisphere contains 

 more than twice as much land as the southern. If the earth be 

 divided into two hemispheres, one with its center in England, and 

 the other in New Zealand (Fig. 2), the first would contain about 



Mountains 

 Plateau 



Fig. 1. Diagram to show the distinction between an elevated continental 

 area and an ocean basin. The steep slope (much exaggerated) at the 

 left of the ocean basin is the line of contact between the two, and is the 

 real border of the continental area. The ocean covers the lower part of 

 the continental tract called the continental shelf. The diagram also 

 shows the general relation between low mountains, such as the Appala- 

 chians, a low plateau, and a coastal plain. The continental shelf is a 

 continuation of the coastal plain. 



six-sevenths of all the land, and might be called the land hemisphere, 

 while the other would contain only about one-seventh of the land, 

 and might be called the water hemisphere. Even in the land hemi- 

 sphere, however, the water would cover rather more than half the 

 surface, while in the water hemisphere it would cover about fourteen- 

 fifteenths of it. Since the northern hemisphere contains two-thirds 

 of the land, and a still larger proportion of the productive land, it 

 has always supported a vast majority of the human race. 



Origin of relief features of the first order. The ocean basins and 

 the continental platforms seem to have been very much as they now 

 are, for millions of years. It is probable that the ocean basins have 

 sunk below the continents, rather than that the continents have 

 been raised above the ocean basins. The reason for this belief is 

 that the earth is cooling, and therefore shrinking, and shrinking 

 means that the outside is getting nearer to the center. This must 

 result in a sinking of the surface, in some places at least. 



Past changes. If the sea bottom were to sink now, the ocean 

 basins would hold more, and some of the sea-water would be drawn 



