5IO JOSEPH BARRELL 



than a hundred kilometers, for there the temperatures are presum- 

 ably above those which under the conditions of freedom from pres- 

 sure at the surface of the earth produce dry fusion. Occluded 

 gases, furthermore, are held beyond possibility of escape. 



The strength of the crust is dependent consequently upon four- 

 fold conditions — the nature of the material, the cubic compression, 

 the relation of temperature to the point of fusion, and the rapidity 

 of the application of the stress. These factors are all variable with 

 time and place. How variable will be seen upon further consider- 

 ation in the following paragraphs. 



The influence of the nature of the material is seen when it is 

 noted that granite is only about one-half as rigid as the basic rocks, 

 although it is not less strong. Consequently, regional stress coming 

 upon a complex of two such rocks will elastically deform the granite 

 more readily, a greater stress will be thrown upon the basic rocks, 

 and since their elastic limit is not correspondingly higher they 

 should begin to yield by flow or fracture before the more phant 

 rocks had reached their limit. The general conclusion is that 

 a movement of compression in the earth's crust must necessarily 

 give rise to unequal strains and concentration of stress, as well 

 from variations in chemical composition as from variations in 

 structure. The local stress may rise far higher than the general 

 regional stress. 



As to the second factor, during the progress of normal fault- 

 ing the horizontal compressive stress in the crust is less than the 

 vertical stress due to weight. During the progress of folding and 

 mashing, on the contrary, the horizontal stresses become far 

 higher. But the least of the three principal stresses determines 

 the amount of cubic compression; the difference between the 

 greatest and least stresses determines, on the contrary, the amount 

 and direction of the strain upon the rigidity of the rock. Thus it is 

 seen that both the cubic compression and the stress-difference vary 

 with the amount and kind of forces. 



It is temperature, however, which is probably the most variable 

 of these factors. Igneous activity brings the temperatures of the 

 greater depths comparatively near to the surface and must produce 



