302 B. WILLIS DISCOIDAL STRUCTURE OF THE LITHOSPHERB 



Plate 12. — Bathymetric Map of the Atlantic Ocean 



Plate 12 is a copy of the bathymetric map of the Atlantic Ocean, by Dr. 

 M. Groll, 1912. The shading represents different depths, and emphasis is 

 placed on depths greater than 5,000 meters, in order to bring out the major 

 deeps. 



The darker areas represent disks, the lighter areas inter disks, under the 

 discoidal hypothesis. 



Plate IS.— Horizontal Stresses and sti^ess Differences between lighter and 



heavier Columns 



The diagram in plate 13 represents two columns (outlined under the words 

 "Light" and "Heavy") which are supposed to be in isostatic balance with each 

 other at the depth of 100 miles and to differ 5 per cent in density. The light 

 column is 100 miles high and the heavy column 95 miles. Each column presses 

 against the other with a stress which is arbitrarily represented by the hori- 

 zontal shading. By hypothesis the lateral stresses are equal at the depth of 

 100 miles, where the two columns are in equilibrium. AB is therefore equal 

 to AC. At any level above the level BO, such as YZ, the stress from the 

 lighter column toward the heavier is the greater; thus XY is greater than 

 XZ. At any level below BC the reverse is true — that is, MN is less than ML. 



Whatever the difference in surface level of the two columns, that we assume, 

 there will always be a level corresponding to BC, at which the weights of the 

 columns will be equal ; and the relations of lateral stress above and below any 

 such level of equilibrium will be those shown in the diagram. 



