THE STRENGTH OF THE EARTH'S CRUST 223 



rocks. The latter consequently tend to stand above the regional 

 levels. They therefore possess surficial excess both of density 

 and volume. 



The average thickness of sedimentary rocks if spread uniformly 

 over the globe is thought to be between 2,000 and 2,500 ft.^ Over 

 the pre-Cambrian areas it must average much less; over the areas 

 of later formations much more. Under the Cenozoic stations 

 assume: 



1,000 ft. of sediments at density 2 . 40 to 2 . 50 



4,000 ft. of sediments at density 2 . 50 to 2 . 60 



Giving a total of 5,000 ft. at density 2 . 48 to 2 . 58 



With a deficiency of density of. o. 19 to o 09 



Under the pre-Cambrian stations assume: 



5, 000 ft. of crystalline rock at density 2 . 75 to 2 . 80 



An excess of density of 0.08 to o. 13 



This does not involve the improbable assumption that below the 

 outer 5,000 feet of crystalline rock of density 2.75 to 2 . 80 the den- 

 sity suddenly decreases to 2.67 and then remains constant through- 

 out the zone of compensation. The vertical density gradient, if 

 uniform for all points, has but little effect, it being the horizontal 

 variations of density which enter into the problem of isostasy. To 

 maintain conformity with Hayford's figures, therefore, the density 

 2.67 will be frequently assumed as the mean density of the litho- 

 sphere, although the previous discussion shows that it cannot be 

 assumed as the density of the outer mile of crystalline rocks when 

 comparing these to the mile of sedimentary rocks taken as the mean 

 depth underlying the Cenozoic stations. 



In comparison with this thickness of 5,000 ft. the average area 

 of formations is very great. A plane sheet of rock 100 ft. thick and 

 of density 2.67, if of indefinite extent, will produce an anomaly of 

 0.0034 dyne upon a point outside of it, irrespective of the distance 

 to that point. This theory may be applied without gross error 

 to the relation of surface geologic formations to anomalies. If 

 this unit mass be expanded from 100 to 5,000 ft. thickness, the 



' F. W. Clark, "Data of Geochemistry," Bull. 4gi, U.S. Geol. Sun., 191 1, p. 30. 



