124 THE FLOOR OF THE OCEAN 



reported off six continents and in the three principal oceans, 

 and it is highly probable that each canyoned region is also in- 

 tensely furrowed. The map of Figure 70, drawn by Professor 

 Shepard, indicates with 101 short lines an equal number of 

 canyons already discovered, though adequate sounding of the 

 continental slopes has only just begun. 6 We see that the dis- 

 tribution of the canyons is world-wide — a fact of great impor- 

 tance for any theory of origin for the great valleys. 



Some of the foreign canyons are approximately en axe with 

 master rivers of the respective continents. Examples are those 

 of the Congo, Ganges, and Indus regions. However, in most 

 cases there is no manifest relation of the kind. 



The floors of the canyons have been dredged and also sam- 

 pled from cores trapped in steel tubes which were driven 

 vertically into the floors. From his pioneer work with these 

 methods, Dr. H. C. Stetson has been able to show that the can- 

 yons off the east coast of the United States are being slowly 

 filled with fine mud. The mud contains the minute shells of 

 foraminifera belonging to species now living in the overlying 

 water. Some inches below the surface of that mud, shells of 

 foraminifera now living farther north, in much colder water, 

 were found. It has been reasonably suggested that this deeper 

 mud was deposited during the last cold stage of the Glacial 

 Period, 20,000 to 40,000 years ago. If such be the fact, Stetson's 

 investigation would lead to two other conclusions: first, that 

 the principal furrowing was accomplished before the last ice- 

 cap began to melt away because of a permanent amelioration 

 of climate; secondly, that under present conditions, the waves 

 and currents of the Atlantic are tending to fill canyons and 

 furrows with sediment. Although the rate of filling is small, 

 its continuation through millions of years would much reduce 

 the ruggedness of the continental slope. Hence it appears that 

 the furrowing was either initiated or accentuated in recent 



