546 GEOLOGY. 



platforms are but the surface forms of great segments of the hthosphere, 

 all of which crowd toward the center, the stronger and heavier seg- 

 ments taking precedence and squeezing the weaker and lighter ones 

 between them. The area of the more depressed or master segments 

 is almost exactly twice that of the protruding or squeezed ones. This 

 estimate includes in the latter about 10,000,000 square miles now 

 covered with shallow water. The volume of the hydrosphere is a little 

 too great for the true basins, and it runs over, covering the borders of 

 the continents. The amount of the overflow fluctuates from time to 

 time, and may be neglected in a study of the movements and deforma- 

 tions of the hthosphere. 



The squeezed segments. — The great protruding segments show 

 a tendency toward rude triangularity. They are (1) the Eurasian, now 

 strongly ridged on the south and east, and relatively flat on the north- 

 west ; (2) the African, rather strongly ridged on the east, but less abruptly 

 elevated on the west and north; (3) the North American, now strongly 

 ridged on the west, more gently on the east, and relatively flat at the 

 north and in the interior; (4) the South American, strongly ridged on 

 the west and somewhat on the northeast and southeast. 



The foregoing form the major group. The minor group embraces 

 (5) the Antarctic segment, not as yet sufficiently kno^vn to be well 

 defined, and (6) the Austrahan, broadly reniform rather than triangular. 

 To these are perhaps to be added (7) the largely submerged platform 

 that stretches from Sumatra and Java on the southwest to the Philip- 

 pines on the northeast, and is attached to India on the northwest; and 

 (8) Greenland, which, though closely associated with North America, 

 is partially separated by a rather deep depression. 



The depressed or master segments. — The great sunken segments show 

 a tendenc}^ to assume roughly polygonal, rather than triangular, forms. 

 This accords with the primary place assigned them, since, in a spherical 

 surface divided into larger and smaller segments, the major parts should 

 be polygonal while the minor residual segments are more Tkely to be 

 triangular. The major segments are (1) the Pacific, (2) the Indian, 

 (3) the North Atlantic, and (4) the South Atlantic. These form the 

 principal group, while (5) the Arctic deeps (not including the shallow 

 epicontinental portions), (6) the Mediterranean, (7) the Caribbean, and 

 (8) the chain of deep pits between the Philippine ridge and the Bornean 

 platform, constitute a subordinate group. 



