OCEANOGRAPHY IN THE UNITED STATES 49 



For more intensive study of the bottom sediments, coring devices 

 are used. Studies of core samples disclose a wealth of important 

 scientific data from the 72 percent of the surface of the earth which 

 is covered with water and which, until recently, has remained hidden 

 from the investigator. The additional fact that the approximate 

 limits of the ocean basins are believed to be unchanged since early 

 geologic times also makes studies of these areas of scientific impor- 

 tance. The sedimentary layers have been deposited in historic se- 

 quence. In the middle of the ocean basins they remain undisturbed 

 and have been deposited very slowly, so that a vertical core a few feet 

 long represents a very long period of time. 



Tlie geologist obtains important information from the character 

 and thickness of the successive layers. From core samples an idea 

 may be obtained of variations in the depth of the ocean in the past 

 and also changes in climatic conditions from a study of the fossils of 

 microscopic animals or foraminifera. 



Many minute particles of magnetic materials that have settled to 

 the bottom are found in bottom core samples, oriented along the line 

 of magnetic force. A study of these minute particles, with delicate 

 electric apparatus, to determine the shift in orientation through the 

 length of the core, reveals the cyclic change of the lines of ma^ietic 

 force throughout the period of time represented by the deposits in the 

 core. 



Every continent rests on a so-called submarine base which extends 

 seaward at varying distances from shore. To this submerged exten- 

 sion of the visible continent has been given the name Continental 

 Shelf. More specifically, it may be defined as the submerged margin 

 of a continent, which slopes to seaward to a point where a substantial 

 break in grade occurs. Or if we approach the matter from seaward 

 rather than from landward it would be the first well-defined rise 

 from the ocean floor, which in the majority of instances would be in 

 the nature of the 1,000-f athom curve. 



The Continental Shelf is not everywhere of uniform width. In 

 the United States it varies from a few miles oflP parts of California to 

 over 200 miles off Cape Cod. In parts of the Gulf of Mexico it has a 

 width of over 200 miles. 



The Continental Shelf comprises an area of more than 300,000 

 square miles in continental United States and 550,000 square miles 

 in Alaska. 



In recent years, the Continental Shelf has assumed increasing im- 

 portance with the discovery of oil and gas. A littoral nation's right 

 to explore and exploit the natural resources of the subsoil and sea- 

 bed of the Continental Shelf and beyond to its limit of competence 

 has been given international approval at the recent United Nation's 

 Conference on the Law of the Sea. 



The Coast and Geodetic Survey has been working in the area of 

 the Shelf for many years in furtherance of its statutory responsibil- 

 ity of providing nautical charts and related information for safe 

 navigation. But many byproducts have resulted therefrom of inter- 

 est both to the scientist and the engineer and the public generally. 

 Detailed surveys of these areas have in the past opened up new fish- 

 ing banks to commercial exploitation, and have furnished evidence of 

 potential oil reserves in the offshore areas. Additional surveys (hy- 

 drographic and oceanographic) may open up new sources of food 



