AN OUTLINE OF GEOPHYSICAL-CHEMICAL PROBLEMS. 1 



By Robert B. Sosman, 

 Geophysical Laboratory, Carnegie Institution of Washington. 



The subject-matter of geophysical-chemistry may be defined as 

 "the physical properties and chemical reactions of the substances 

 and aggregates that make up the earth." It may therefore be 

 roughly divided into two parts: A. Properties and reactions of 

 materials accessible at the earth's surface. B. Properties and re- 

 actions of materials in the earth's interior. 



Each of these may again be subdivided as follows : 



1. Properties and reactions of individual chemical substances; for 

 example, the silicate minerals. 



2. Properties and reactions of aggregates; for example, oceanic 

 water, silicate rocks. 



3. Properties and reactions of larger units of matter; for example, 

 glaciers, batholiths. 



A. MATERIALS AT THE EARTH'S SURFACE. 



CHEMICAL SUBSTANCES. 



A relatively small number of " common " oxides serves to make up 

 practically 98 per cent by weight of the outer 10 miles of the litho- 

 sphere. All the other elements and compounds known to chemistry 

 are included in the remaining 2 per cent. From the geochemical 

 standpoint, therefore, we may divide chemical substances into two 

 classes, " abundant " and " rare." 



The " abundant " oxides are, in the order given by averages of a 

 great many analyses of terrestrial rocks, as follows : 

 SiO, About 60 per cent by weight. 

 Al 2 6 3 About 15 per cent by weight. 



' About 6 per cent by weight, 



Fe,0 ? 



* Reprinted by permission from the Proceedings of the National Academy of Sciences, 

 vol. 6, No. 10, pp. 592-601. October, 1920. 



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