loo PRINCIPLES OF STRATIGRAPHY 



it is on this relationship that the other characteristics are, in a large 

 measure, dependent. As shown in the introductory chapter, the 

 most natural view to take of the land mass as a whole is that it 

 constitutes three continental blocks— the American; the Old World, 

 comprising Europe, Asia, Africa, and Australia; and the Antarctic. 

 These three great blocks are separated one from the other by the 

 great divisions of the sea, the oceans, which, therefore, have an 

 intercontinental location. These continental blocks and dividing 

 oceans have certainly been distinct since the Mesozoic, and it is 

 prol)able that .they have been distinct since much earlier, possibly 

 Palaeozoic, time. It is, of course, true that America has at dififerent 

 times been united with Europe or with Asia on the north, and 

 with Antarctica on the south, either by land bridges or by shallow 

 submarine banks. But this does not, therefore, destroy the essen- 

 tial independence of the three great blocks. If, then, we regard 

 the oceans as the intercontinental bodies of sea water, and consider 

 America and the Old World as distinct blocks, we have the follow- 

 ing four: 



I. Intercontinental Sea.s or Oceans. 



1. Pacific — between all three blocks, with a superficial area * of i66 



million square kilometers. 



2. Atlantic — between all three blocks, W'ith a superficial area * of 82 



million square kilometers. 



3. Indian — between Old World and Antarctica, with a superficial 



area of 73 million square kilometers. 



4. Arctic — between Old and New World, with a superficial area of 



about 14 million square kilometers.* 



If, on the other hand, we consider, with Wagner, Penck, and 

 others, the Old and New World together as one block, we must take 

 the Arctic Sea from the list of oceans and place it as an intra- 

 continental body among the mediterraneans. 



Bathymetric Zones of the Sea. A strip of shallow sea sur- 

 rounds each of the four great oceans and constitutes the belt of 

 transition from the sea to the land. This is the littoral belt, the 

 width of which corresponds to the width of the continental shelf. 

 Similarly, a littoral belt outlines each group of oceanic islands, 

 though this is generally very narrow. The depth of the littoral belt 

 does not, perhaps, greatly exceed 200 meters (roughly, 100 fathoms), 

 and it corresponds very closely with the depth to which sunlight 



* Exclusive of marginal bodies. 



