EARTH S CRUST — WASHINGTON. 



307 



1. Earth (5159 analyses). Density=2.77. Elevation = +2,252 feet. 



2. North America (1709 analyses). Density=2.75. Elevation= 1,888 feet. 



3. South America (138 analyses). Density=2.72. Elevation= 2,078 feet. 



4. Europe (1985 analyses). Densitv=2.75. Elevation= 939 feet. 



5. Asia (114 analyses). Density=2.72. Elevation=3,189feet. 



6. Africa (223 analyses). Density=2.77. Elevation= 2,021 feet. 



7. Australia (287 analyses). Density=2.79. Ele.vation= 805 feet. 



8. Antarctica (103 analyses). Densitv=2.79. Elevation=? feet. 



9. Atlantic floor (56 analyses). Density=2.85. Depth = -13,500 feet. 

 10. Pacific floor (72 analyses). Density=2.89. Depth = -14,820 feet. 



Before we discuss the densities it will be well to examine the aver- 

 age chemical compositions of the different continents and ocean floors, 

 the data for which are given in Table IV. It will be seen that they 

 vary considerably the one from the other, as well as from the general 

 average of the earth's crust. Taking, for example, silica, the most 

 abundant constituent, its percentages for North and South America, 

 and especially for Asia, are decidedly above that of the earth's crust 

 as a whole, while those for Europe and Australia are only slightly 

 above this. On the other hand, the silica percentages for Africa and 

 Antarctica, and still more for the Atlantic and the Pacific floors, 

 are very notably lower. 36 There is little difference, comparatively, in 

 the figures for alumina, the alkalies, and the minor constituents, but 

 those for the iron oxides, magnesia, and lime are distinctly lower in 

 those cases where silica is higher and higher where this is lower. 



The continental and oceanic averages shown above represent, in 

 fact, different comagmatic regions on a large scale, but in concise 

 form. Though the data for some of them — as Asia, South America, 

 Africa, and Antarctica — are not numerous enough to be wholly satis- 

 factory, yet there would seem to be no valid reason for doubting 

 that, taken as representing broadly the general chemical composi- 

 tions of the larger structural divisions of the earth's surface, they 

 may safely be assumed to give us a fairly trustworthy idea of the 

 relations between them. . In any case, they are the only large body 

 of data that we have available, so let us use them Drovisionally and 

 see to what results their consideration may lead. 



Before doing this, however, it will be as well to devote a few words 

 to the average density of the crust as a whole, as this is an important 



86 If the analyses of the rarer rocks are disregarded, 

 about 3.05 and that of the Pacific is about 3.10. 



the density of the Atlantic floor Is 



