276 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 3 8 



relatively small scale. This matter concerns us here only insofar as 

 the form and covering of the ocean floors may throw light on the 

 controversy. 



The variation in depth of the oceans over the surface of the globe 

 is familiar to all. In their main outline, the bathymetrical contours 

 have long since been plotted, and the work of recent deep-sea ex- 

 ploration has only necessitated modification in detail of such contours. 

 The broad distribution of deep-sea deposits — the organic oozes and 

 red clay — in relation to depth is also a matter of general knowledge. 

 The character of these deposits and the areas and depths of the floors 

 where they occur was first made known from the classical investiga- 

 tions of the Challenger expedition in 1880, although earlier expeditions 

 had foreshadowed some of the results. Since then numerous scientific 

 explorations have been made, and have duly been referred to in text- 

 books of oceanography. 



With certain exceptions, the types of deposits found on the ocean 

 floor are related to the depth, the distribution of the various types 

 corresponding with different submarine levels. A clear picture of the 

 respective areas in relation to the heights or depths is furnished by 

 what is known as the hypsographical curve of the earth's surface. 

 From this diagram, in which areas are plotted as abscissae and heights 

 as ordinates, it becomes evident that two levels are most widespread. 

 The first is a land level consisting of the greater part of the conti- 

 nents (all but about 9 percent, which constitute the "mountainous" 

 areas) together with a continental shelf that is submerged beneath 

 the sea to an eventual depth of about 100 fathoms (nearly 200 

 meters) at its margin. The second is the deep-sea level, which in- 

 cludes most of the ocean floor; this, if regarded as lying between 

 1,400 and 2,800 fathoms (2% and 5 kilometers) in depth, occupies 

 about two-fifths of the earth's surface. The ill-defined stage con- 

 necting these levels, that is, the continental slope, is of small extent, 

 possibly 8 percent of the earth's surface, and the deeps or sinks in 

 the ocean floor lying below 3,000 fathoms in depth (5% kilometers) 

 constitute only about 4 percent of the surface, although this propor- 

 tion may prove to be slightly greater when more records of depths 

 are obtained. The deep-sea area, including the deeps, thus com- 

 prises some 115 million square miles (295 square kilometers), or more 

 than half the earth's surface ; the character of its surface and covering 

 would therefore appear to merit greater attention than they have 

 hitherto been given, especially when we recall the vast literature 

 concerned with the geology of the continental areas of the globe. 



Successive expeditions have added considerably to the number 

 of sounding stations originally established by the Challenger, and 

 we now realize that the ocean floor has topographical irregularities 



