THE STRENGTH OF THE EARTH'S CRUST— Concluded 



JOSEPH BARRELL 



New Haven, Connecticut 



PART VIII. PHYSICAL CONDITIONS CONTROLLING THE 

 NATURE OF LITHOSPHERE AND ASTHENOSPHERE 



SECTION B 

 RELATIONS WITH OTHER FIELDS OF GEOPHYSICS 



Erroneous Conclusions Reached by the Rectilinear Projection 



or Surface Conditions 499 



The Evidence of Tides on Rigidity and Strength . . . . 504 



The Evidence of Earthquake Waves on Rigidity and Density . 506 



High, but Variable, Elastic Limit within the Upper Lithosphere 509 



Modes of Lithospheric Yielding and Their Relation to Strength 512 



RELATIONS WITH OTHER FIELDS OF GEOPHYSICS 



Erroneous Conclusions Reached by the Rectilinear Projection 

 OF Surface Conditions 



Geologists early became aware that temperature increased with 

 depth. Projecting this gradient as a straight line indicated that 

 at no great depth the temperature was sufficiently high to melt all 

 rocks and, in testimony, volcanoes brought such melted rocks to 

 the surface. The earth was consequently looked upon as a molten 

 or even gaseous body enveloped by a thin crust of solid rock. The 

 logic of this conclusion seemed incontrovertible and moreover it 

 was in accord with the simpler expectations from the nebular 

 hypothesis. Nevertheless, direct and positive evidence from sev- 

 eral independent sources has forced on geologists the belief that 

 the earth is not only solid throughout, but, as a whole, is more 

 rigid than steel. Slowly and with difficulty the older view has 

 therefore had to be abandoned. Yet it continually recurs in one 

 form or another, advocated chiefly by writers who see the direction 

 in which the surface evidence of temperature gradient leads, who 

 regard it as compulsory, and who do not recognize or give equal 



499 



