THE STRENGTH OF THE EARTH'S CRUST 215 



rocks contain both igneous and sedimentary formations, but the 

 considerable iron ore and metamorphic nature would bring the 

 specific gravity of the sediments somewhat above the average 

 of 2 . 70 for non-porous sediments. Broad areas of pre-Cambrian 

 probably range therefore between 2.75 and 3 .00 in specific gravity. 

 More limited areas, because of a predominance of granite and 

 quartzite, may range as low as 2 . 70. About 2 . 67, however, would 

 be a minimum. 



As these are merely averages it is better in basing calculations 

 upon them to assume a certain range in density for each figure and 

 to obtain thus a knowledge of the influence of reasonable variations 

 upon the results. The data may then be tabulated as follows: 



TABLE IX 



Estimated Mean Specific Gravities of Geologic Formations 



Pre-Cambrian 2 . 75-2 . 80 



Paleozoic and Mesozoic 2 . 50-2 . 60 



Cenozoic 2 . 40-2 . 50 



The range in these specific gravities shows the necessity of con- 

 sidering them in all refined calculations on the anomalies of gravity. 

 In place, however, of using a mean density figure for all stations on 

 formations of a certain geologic age, it would be of much more 

 value to have measurements of the actual surface densities occurring 

 in each area; also estimates by geologists, based on geologic 

 structure and these surface measurements, of the densities extend- 

 ing to the base of the sedimentary rocks of each locality. 



It seems probable from the mean density of 2 . 80 obtained for 

 igneous rocks that the density of 2.67 used by geodesists for the 

 mean density of the zone of compensation is too low. If any 

 variation from the average composition takes place with depth 

 within the limits of 76 miles, it is likely to be a variation toward 

 more basic and heavier rocks. Assuming, however, an average 

 uniformity of chemical composition, the opposing effects of tem- 

 perature and pressure remain to be considered. Using the coeffi- 

 cient of expansion of the average igneous rock computed by W. H. 

 Emmons,^ 0.000,019,9 ^or i°C., and a temperature gradient of 



' Chamberlin and Salisbury, Geology, I (1904), 547. 



