THE EARTH AS A WHOLE 9 



In some respects, as has already been stated, it might serve our 

 purj)ose better to consider the main land groups as three separate 

 continental blocks, namely ; the Old World block, comprising the 

 continents of Eurasia, Africa and Australia ; the New World block 

 or the two Americas, and the Antarctic block. On the basis of 

 such division we may consider that we have four great oceans 

 separating these blocks one from another, namely, the Atlantic, the 

 Pacific and the Arctic * separating the Old and the New World 

 blocks, and the Indian Ocean separating the Old World from the 

 Antarctic block. These oceans would then be the intercontinental 

 divisions of the sea, while the divisions of these masses into conti- 

 nents, as ordinarily understood, would be accomplished by mediter- 

 raneans, which may then be regarded as i^irocontinental in char- 

 acter. 



Isostasy. Carrying out this idea, we have to consider the sea 



Sea-level. 



Fig. 2. Diagram illustrating the relationships between the denser suboceanic 

 crustal blocks A and D and the light or terrestrial crustal blocks 

 B, C, when in static equilibrium. (After Penck. ) 



as divided into four great blocks corresponding to the four oceans, 

 including the Arctic. If we divide the crust into continental and 

 suboceanic masses or crustal blocks, we must consider that these 

 masses are in static equilibrium, due to the greater density of the 

 material of the suboceanic masses, as shown by pendulum experi- 

 ments. Penck {22 -.12^-126) illustrates these relations by compar- 

 ing these masses with boards of equal thickness, but of diiTerent 

 weights floating upon water. Those of heavier wood will sink 

 deeper than those of lighter wood, and may be taken to represent 

 the suboceanic masses. This static equilibrium of the blocks of the 

 earth's crust constitutes the phenomenon of isostasy, and indicates 



* It must, on the other hand, be considered that the Arctic Ocean has the 

 character of a mediterranean, in that its abyssal portion is everywhere separated 

 from that of the sea as a whole. In fact, at the present time, not only is the 

 2,400-meter line continuous within the Arctic, but this is equally true of the 

 1,000-meter line, the depths between North America, Greenland, Iceland, and 

 Northern Europe not going below this. That the Arctic Ocean is one independent 

 depressed earth block similar to those forming the three great oceans can hardly 

 be doubted. The peculiarities which differentiate it from the other oceans are 

 due largely to its location at the earth's axis, and to its extensive covering of ice. 



