42 JOSEPH BARRELL 



CHARACTER OF THE CURVE OF STRENGTH 



In curve F' of Fig. i8 is shown the nature of the curve of 

 strength as suggested by the geodetic evidence from the United 

 States. In curve F" is shown the nature of the curve as suggested 

 by the departures from isostasy exhibited by the great mountain 

 axes and possibly by the continental ice sheets. These curves 

 may be taken as showing the value of the elastic limit at various 

 depths for permanent stresses. With varying geologic conditions, 

 especially those connected with rising magmas and their emana- 

 tions, the curve of strength must vary widely, and furthermore no 

 very close paralleKsm of strength-curve and stress-curve is to be 

 expected. These curves, therefore, are intended to bring out 

 general relations; they are of qualitative, not quantitative value. 

 The drawing of curve F" somewhat inside of curve E means that 

 below the point of maximum stress in E, as given for a homogeneous 

 elastic earth, the stress is assumed as somewhat greater than the 

 crust at those levels can sustain. Upon the development of this 

 load plastic flow in these deeper levels would take place main- 

 taining the stress within the strength curve for each level; the 

 crust above would come to act to some extent as a bending plate, 

 the stresses within it would increase, chiefly within the upper and 

 lower portions. This added strain would compensate for the yield- 

 ing below. For the reasons discussed previously, however, show- 

 ing the structural weakness of the lithosphere as a beam, this action, 

 it is thought, could not go very far, and, in consequence, the loads 

 on the lithosphere are essentially such as to give stresses contained 

 within it, distributed according to Darwin's law. The preceding 

 deals only with that part of the curve of strength which marks the 

 gradation from lithosphere to centrosphere. The relations of this 

 part to those above and below need still to be considered. 



The highest stress found for the loads regarded as harmonic 

 waves was for the great folds on the floor of the Pacific Ocean. 

 These were taken as equivalent to harmonic waves of rock of 

 density 2.67, 400km. in wave-length, and 4,000m. in amplitude. 

 But even these folds give a maximum stress of only 393 kg. 

 per sq. cm. (5,590 pounds per square inch), and this at a 

 depth of 64 km. At the surface strong limestone or granite can 



