326 PHYSIOGRAPHY 



and such as those about some of the pronounced deeps; (3) valley- 

 like depressions, especially on the continental shelves; (4) great 

 ridges somewhat like the mountain ridges of the land; and (5) 

 broad, plateau-like swells, much above their surroundings, and 

 over which the water is not very deep. 



1. Volcanic cones are more numerous in the Pacific Ocean than 

 elsewhere, and more numerous in its deeper western part than in 

 its shallower eastern part. They seem to rise abruptly, but their 

 slopes are much less steep than they seem. The upper parts of 

 volcanic islands rarely have a slope of more than 30, and their 

 lower parts rarely more than 6 to 10. Below the sea the slope 

 is still gentler, rarely more than 3, or 1 mile in 20 (Fig. 263). 



2. Though the slopes of the bottom at the edges of the conti- 

 nental shelves and above the deeps are steep, as slopes in the ocean 



Fig. 263. Diagram illustrating a slope corresponding to 1 : 20. 



bottom go, they are much less steep than many slopes on land. 

 A slope of 1 mile in 8 is rare, and a slope of 1 mile in 20 (Fig. 263) 

 can hardly be said to be common. Such a slope would make a steep 

 railway grade. 



3. On many continental shelves there are valleys which are 

 much like river valleys. Many of them are continuations of valleys 

 on land. Thus the Hudson, the Delaware, the Susquehanna, the 

 St. Lawrence, and other valleys are continued out under the sea. 

 Submerged valleys are commonly thought to have been formed 

 by rivers when the areas where they occur were land. 



4. Examples of mountain-like swells are furnished by Cuba and 

 the adjacent islands, which are really the crests of a great mountain 

 system rising from deep water. 



5. An example of the plateau type of elevation is the Dolphin 

 Ridge of the Atlantic (Fig. 264). This broad, low swell extends 

 to latitude 40 S., and divides the basin of the Atlantic into two 

 troughs, the one to the east and the other to the west, where the 

 water is much deeper than over the ridge itself. In the Southern 

 Pacific, many volcanic islands rise from submerged plateaus. 



