328 Status of the Theory of Isostasy. 



were assumed to extend to the center of the earth. A 

 still larger proportion of the compensation would lie 

 above the center of gravity, and the portion of the curve 

 below the center of gravity would become much more 

 attenuated than in curve D, fig. 5. At depths below the 

 center of gravity, the compensation would approach the 

 vanishing mass of a mathematical line, and as such 

 could be extended indefinitely. Even for curve D, in 

 fact, where the limiting depth is taken as only 178-6 miles, 

 but 5 per cent; of the compensation lies below a depth 

 of 107 miles. For a greater assumed extension in depth, 

 a still smaller proportion of the compensation would be 

 found to lie below a depth of 107 miles. 



The geodetic data which gave the solutions shown in 

 figure 5 could be made to yield an infinite number of 

 solutions, but beyond certain limits these would have no 

 geologic meanings and become simply mathematical ex- 

 ercises. It is appropriate next to inquire regarding the 

 limits within which lie the geologic possibilities. 



Disregarding the small amounts of unconsolidated 

 sediments or molten magmas which may lie in the crust, 

 the specific gravities of the principal rock types may be 

 taken as follows : 



Rock Specific Gravity 



Granite 2-63-2-75 



Syenite 2-6 -2-8 



Diorite 2-8 -31 



Dolerite 30 -3-3 



Limestone 2-6 -2-8 



Sandstone 2-5 -2-7 



Shale 2-4 -2-8 



Slate About 2-8 



The mean density of the rocks which underlie the con- 

 tinents to a depth of several miles may be taken at 2-70 

 to 2-75 as a minimum. 



The mean elevation of the continents may be taken as 

 2,400 feet, the mean depth of the sea as 13,000 feet. If 

 the sea water is imagined transformed into rock and the 

 bottom of the dry ocean beds raised accordingly, the 

 mean depth of these would become 8,000 feet instead of 

 13,000 feet. The mean relief between continents and 

 ocean basins to be supported by isostatic compensation 

 becomes, consequently, 10,400 feet or 2 miles. Consider- 



