192 The Realm of Nature CHAP. 



10,000 feet deep, is called the Transitional Area. The 

 Abysmal Area, or group of World Hollows, is capacious 

 enough to contain exactly the whole volume of the group 

 of World Ridges made up of the Transitional and Conti- 

 nental Areas. The position of the coast-line or boundary 

 between the Transitional and Continental Areas obviously 

 depends on the volume of the hydrosphere. It is con- 

 venient for most purposes to class the Abysmal and Tran- 

 sitional Areas together as the Bed of the Oceans. In 

 originally proposing this division of the Earth's surface, Dr. 

 Murray took the boundary line between the Transitional and 

 Abysmal areas at the arbitrary depth of 1000 fathoms, or 

 6000 feet below sea-level. 



256. Elevated Half of the Lithosphere. The eleva- 

 tions and depressions of the Earth, although irregular in 

 form and distribution, are arranged with a certain rough 

 symmetry about the poles. A small detached elevation 

 occupying about one-twelfth of the area of the elevated 

 half has its centre within the Antarctic circle, and slopes 

 down gradually on all sides to mean sphere level. The 

 surface of the northern hemisphere is as a whole more 

 elevated than that of the southern. A great Northern 

 Plateau surrounding the pole to a distance of 2000 miles, 

 and broken only by one depression (that of the Norwegian 

 and Arctic Seas), is the centre of a continuous mass com- 

 prising fully nine -tenths of the whole elevated half, and 

 extending toward the south irv two vast World Ridges of 

 unequal size. In reading the following paragraphs the 

 student should refer constantly to the map (Plate XL), and 

 to Plate XIV. on which the line of mean sphere level is 

 depicted. The Western World Ridge stretches from 60 N., 

 where the Polar plateau splits, in a south-easterly direction 

 to the equator, and thence southward, rapidly narrowing, 

 to 60 S. The ridge, nowhere of great width, is narrowest 

 between the Tropic of Cancer and the equator, where three 

 small isolated depressions (the basins of the Caribbean Sea 

 and Gulf of Mexico) nearly sever it. The crest of this 

 ridge forms the connected continents of America. The 

 Eastern World Ridge is of much greater size, and has 



