12 PHYSIOGRAPHIC GEOLOGY. 



17. (3.) Oceanic depression. — (a.) Outline. — The oceanic depres- 

 sion is a vast sunken area, varying in depth from 1000 or less to, 

 probably, 50,000 feet. 



The true outline of the depression is not necessarily identical 

 with the present line of coast. About the continents there is often 

 a region of shallow depths which is only the submerged border of 

 the continent. On the North American coast off New Jersey this 

 submerged border extends out for 80 miles, with a depth, at this 

 distance, of only 600 feet ; and from this line the ocean-basin dips 

 off at a steep angle. The true outline of the basin on this and 

 other coasts is shown by the dotted line on the chart. The slope 

 for the 80 miles is only 1 foot in 700. 



Great Britain is, on the same principle, a part of the European continent : the 

 separating waters are under 600 feet in depth; and a large part of the German 

 Ocean is only 93 feet. The true oceanic outline extends from Southern Norway 

 around by the north of Scotland and southward into the Bay of Biscay. (See 

 the dotted line on the chart.) In a similar manner, the East India Islands, 

 down to a line running by the north of New Guinea and Timor, are a part of 

 Asia, the depth of the seas intermediate seldom exceeding 300 feet; while south 

 of the line mentioned the islands are but fragments of Australia, the water being 

 no deeper than over the submerged Asiatic plateau.*' 



(b.) Depth of the Ocean. — The depth of the ocean in its different 

 parts has not been ascertained. Some deep soundings have been 

 made, and a few of them claim to have reached to a depth of 

 twenty-five to forty-five thousand feet ; but the methods of sound- 

 ing employed have been shown to be unsatisfactory, and the results, 

 therefore, are valueless.! Across from Ireland to Newfoundland, 

 the depth has been found to vary between 10,000 and 15,000 feet. 

 The Gulf of Mexico is known to be from 4000 to 5000 feet in depth. 

 According to calculations on the data furnished by an earthquake- 

 wave which, in 1855, crossed from Simoda, in Japan, to San Fran- 

 cisco, the ocean in that line has an average depth of about 13,000 

 feet. The depth in the northern part of the Pacific and Atlantic 

 may be, therefore, nearly the same. South of this in each it is pro- 

 bably very much greater. 



The mean depth of the oceanic depression is, by estimate, 

 between 15,000 and 20,000 feet. 



(c.) Character of the Oceanic Basins. — To appreciate the oceanic basins 



* Earl, Jour. Indian Arch. [2], ii. 278, and Am. Jour. Sci. [2], xxv. 442. 



-j- Some of the results are as follow : — A sounding by Capt. Ross, 900 m. S.W. 

 of St. Helena, 27,600 feet without bottom ; by Capt. Denham, in 36° 49' S., 37° 

 6' W., 46,236 feet (7706 fathoms) found bottom. 





