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THE FLOOR OF THE OCEAN 



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FIGURE 2. DIAGRAMMATIC CROSS-SECTION OF A CONTINENTAL TERRACE. 



averaging about i in 15. This second belt, now recognized as 

 the flank of the continent, is called the continental slope. Shelf 

 and slope together make the composite surface of a great three- 

 dimensional bench which is conveniently named the continen- 

 tal terrace. Each continent is bordered by its terrace, varying 

 in width along the shores but essentially continuous. The total 

 length of the terraces, the world over, is at least 60,000 miles; 

 their average breadth, measured from shoreline to foot of the 

 continental slope, is roughly 100 miles. 



Observation shows that the continental terrace is composed 

 of two kinds of material — underlying hard rock and an over- 

 lying soft formation. The former represents an offshore belt of 

 the continent where drowned to moderate depth by down- 

 warping or by collapse along steeply inclined fractures or so- 

 called faults. At the seashore one can actually see the surface 

 of the hard-rock continent dip gently under the sandy beach 

 and wave-tossed water. In many cases nubbles or considerable 

 masses of the continental rock reappear in the form of islands 

 or skerries, many miles from shore. Typical examples inter- 

 rupt the flat floor of the Australian shelf, whose seaward limit 

 is near the line of the Great Barrier Reef, made by corals 

 growing up on the shelf. Here many islands of granite and 



