BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 



VOL. 33, PP. 287-302 JUNE 30. 1922 



CONDITION OF THE EARTH'S CRUST AND THE EARLIER 

 AMERICAN GRAVITY OBSERVATIONS 1 



BY GEORGE R. PUTNAM 2 



{Read in full before the Society December 29, 19 ( 21) 



CONTENTS 



Page 



Introduction 287 



Development of the theory of isostasy 288 



The transcontinental gravity measurements of 1894 290 



Hayf ord's reduction of gravity observations 290 



The gravity reductions of 1895 compared with earlier and later methods. 291 



Comparison of results for regional or local compensation 299 



Introduction 



The known facts as to the earth and its structure, beyond the knowl- 

 edge derived from what is visible on its surface, are comparatively few. 

 Its general shape has been determined and its principal dimensions and 

 the proportion of flattening measured. The mean density has been com- 

 puted. Increasing temperature below the surface has been observed. 

 The interior of the earth is estimated to be more rigid than steel, but it 

 yields to great forces. Its elasticity is observed in earth tides and in the 

 transmission of earthquake shocks. Volcanic craters, geysers, and other 

 outlets give information as to subsurface activities and materials. Earth- 

 quakes afford some measure of the more violent adjustments of the crust. 

 The earth is a great magnet. 



The greatest depth to which man has penetrated is but 7,579 feet, 

 about 1/2 700th part of the earth's radius. This limitation and the 

 meager knowledge of the planet most important to man render of special 

 interest any observational means of studying the conditions below the 

 surface. As a by-product of trigonometrical and geodetic surveys of wide 

 extent, planned to determine the size and shape of the earth and to form 



1 Manuscript received by the Secretary of the Society December 15, 1921. 

 This paper is one of a series composing a symposium on isostasy. 



2 Introduced by William Bowie. 



(287) 



