THE CHEMISTRY OF THE EARTH'S CRUST. 1 



By Henby S. Washington, Ph. D., 

 Petrologist, Geophysical Laboratory, Carnegie Institution of Washington. 



INTRODUCTION. 



The term " Crust of the earth " is a heritage from the days when 

 the interior of the earth was generally conceived to be a " sea of 

 molten rock," at an enormously high temperature, covered by a rela- 

 tively thin, solid crust of cooled matter. Various cogent reasons, 

 into the consideration of which we can not enter here, have led to 

 the abandonment of this concept, and we now have reason to hold 

 the following tenets as to the conditions that obtain in the earth's 

 interior : 



1. The interior is essentially — or, at least, behaves essentially like — • 

 a rigid solid, though possibly a certain amount of viscosity may be 

 granted. 



2. It is hot, but of an unknown temperature, and probably in- 

 creases in temperature toward the center, with a gradient that is 

 unknown beyond very moderate depths, and that is probably very 

 different in different places. 



3. It is of a density greater than that of the " crust," inasmuch 

 as the mean density of the earth as a whole is about 5.55, while that 

 of the crust is about 2.77, as will be shown later. 



4. The earth, as a whole, acts in many respects as a magnet, and 

 as the rocks of the crust in general are not notably magnetic, this 

 ma}' be attributed to the characters or composition of the interior 

 materials. 



5. From the study of the propagation of earthquakes we are led 

 to believe that there is a change in the physical properties at about 

 0.5 of the radius in depth, the matter below this not transmitting 

 transverse vibrations. Studies on the compressibility of rocks by 

 Adams and Williamson, in the Geophysical Laboratory, indicate that 

 the high density of the interior can not be explained by compres- 



1 Reprinted by permission from the Journal of the Franklin Institute, December, 1920, 

 based on a paper presented at a meeting of the section of physics and chemistry held 

 Thursday, Mar. 4, 1920. „ 



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