14 PHYSIOGRAPHIC GEOLOGY. 



The islands adjoining the continents are properly portions of the 

 continental regions. Besides the examples mentioned on page 12, 

 Japan and the ranges of islands of eastern Asia are strictly a part of 

 Asia, for they conform in direction to the Asiatic system of heights, 

 and are united to the main by shallow waters. Vancouver's Island 

 and others north are similarly a part of North America ; Chiloe, and 

 the islands south to Caj^e Horn, a j3art of South America ; and so in 

 other cases. 



The body of the continent of Africa lies in those latitudes which 

 arc almost wholly water in the American section, its western expan- 

 sion corresponding to the indentation of the Caribbean Sea and the 

 Gulf of Mexico. 



(c.) Oceanic Islands. — The islands of mid-ocean are in lines, and 

 are properly the summits of submerged mountain-chains. The At- 

 lantic and Indian Oceans are mostly free from them. The Pacific 

 contains about 675, which have, however, an aggregate area of only 

 80,000 square miles. Excluding New Caledonia and some other large 

 islands in its southeastern part, the remaining 600 islands have an area 

 of but 40,000 square miles, or less than that of New York State. The 

 islands stretch off in a train from the Asiatic coast through the tropics 

 in an east-southeast direction, and, soon crossing the equator, lie 

 mostly in the southern tropic. The train extends to Easter Island 

 and Sala-y-Gomez, in longitudes 110° and 105° "W., a distance of 

 8,000 miles. The greatest depth of the ocean should be looked for 

 outside of the limits of this train. 



(d.) Mean elevation. — The mean height of the continents above the 

 sea, exclusive of Australia and Africa, according to an estimate by 

 Humboldt, is about 1,000 feet; and this is probably not far from the 

 truth for all the land of the globe. As the area of the ocean and land 

 is as 8 to 3, if all this land above the present water-level were trans- 

 ferred into the oceans, it would fill them 3-8ths of 1,000 or 375 feet; 

 and, taking the average depth at 15,000 feet, it would take 40 times 

 this amount to fill the oceanic depressions. 



The mean height of the several continents has been stated as fol- 

 lows : Europe, 670 feet; Asia, 1,150; North America, 748 ; South 

 America, 1,132; all America, 930; Europe and Asia, 1,010; Africa, 

 probably about 1,600 feet; and Australia, perhaps 500. It has been 

 estimated that the material of the Pyrenees spread over Europe would 

 raise the surface only 6 feet ; and the Alps, though four times larger 

 in area, only 22 feet. 



The extremes of level in the land, so far as now known, are, 1,300 

 feet below the level of the ocean, at the Dead Sea, and 29,000 feet 

 above it, in Mount Everest of the Himalayas. Both of these points 



