402 II. S. WASHINGTON ISOSTASY AND ROCK DENSITY 



In Table IV are given the results of the computations for the earth as 

 a whole and for those countries which were represented by the most 

 numerous data. It may be considered that the data for the countries not 

 found in the table are insufficient for proper generalizations. 



Table IV. — Average specific (J rarities by the physical Method 



Normative 

 density 

 Specific gravity Altitude Determinations 5 ; i <5v 



Earth 2 . 761 700 1.849 2 . 792 2 . 737 



United States 2 . SOS 760 295 2 . 780 2 . 730 



British Columbia 2 . 760 1.100 49 2 . 806 2 . 749 



Andes 2.598 1,820 39 2.752 2.700 



Great Britain 2 . 827 100 57 2 . 830 2 . 758 



Germany 2.790 300 252 2.814 2.744 



Austria-Hungary 2.785 400 66 2.817 2.744 



Switzerland-Tyrol 2.766 1,550 161 2.784 2.702 



Italy 2.680 500 146 2.763 2.727 



Balkania 2.614 800 43 2.738 2.691 



Malaysia 2.686 650 57 2.774 2.721 



Australia 2.775 350 238 2.814 2.747 



Atlantic islands 2. 861 —1,830 34 2 . 888 2 . 810 



The average specific gravities with water neglected, (8 a ) as determined 

 by the physical method are uniformly lower than the densities determined 

 by the normative method, with the single exception of the United States. 

 For the Andes and Balkania the values are so low that this may' be 

 ascribed with confidence to the inclusion of many glassy lavas among the 

 specimens examined, and the same is probably true (but to a less degree) 

 as regards Italy and Malaysia. The normative densities reckoned with 

 water, 8 W , are generally lower than the physical densities. 



But, however unsatisfactory these figures may be, the interesting fact 

 appears on confrontation of the specific gravities with the corresponding 

 altitudes, that the general law of inverse relation between density and 

 altitude holds good. The correspondence is not so clear as with the other 

 method, and the law is partly masked by the presence ot' many glassy 

 rocks, but the general results are in harmony with the conclusions arrived 

 at in the preceding pages. 



The isopiestic Level 



According to the theory of isostasy. the various columns of different 

 densities and heights of which the crust is assumed to be composed bal- 

 ance each other or are of equal weight at a certain depth. This depth is 

 called the "depth of compensation*' by Hayford and Bowie. The term 

 ''isopiestic level" is suggested here to denote this subcriistal region because 



