442 JOSEPH BARRELL 



the existence of stress-difference, through a depth greater still. 

 The beginning of permanent contact of the two curves, if this is 

 the cause of the disappearance of strength, should be as much as 

 300 km. deep and extend through some hundreds of kilometers. 



A rectilinear projection downward of the temperature gradient 

 observed at the surface would reach the fusion temperature of rocks 

 at a depth of about 50 km. There must be consequently a marked 

 curvature of the temperature gradient if the temperature and 

 fusion curves do not meet short of 300 km. This curvature implies 

 that near the surface there is either a greater quantity of heat flow- 

 ing outward by conduction or that the conductivity of rock near the 

 surface is very greatly decreased. But such a very great decrease 

 in conductivity making for a higher temperature gradient finds no 

 supporting evidence. On the other hand, a greater outward flow 

 by conduction of heat near the surface may be due to the con- 

 tinued generation of heat by radioactivity to a greater degree than 

 below; or also to a rise of magmas from the asthenosphere. Mag- 

 mas which never reach the surface would bring heat by a convective 

 process directly into the outer crust. From there the heat, slowly 

 diffused upward by conduction, would increase the temperature 

 gradient in the outermost part of the lithosphere. It is this factor 

 especially which the argument of the present chapter emphasizes. 



It is only within the present generation that general recognition 

 has been given to the intrusive nature of the abyssal igneous rocks. 

 They are now generally regarded as risen from the depths. Their 

 action has been to break through and engulf the foundations of the 

 ancient crust. 



This process of batholithic invasion seems to be recurrent and 

 widespread, though rise into the outer crust is restricted to the 

 crises of diastrophism and usually reaches levels exposed to erosion 

 only along the lines of mountain systems. The stores of heat 

 brought up from the greater depths would be held in the crust, 

 especially in its deeper parts, for geologic ages, blurring out in the 

 course of time by conduction and creating a false appearance of 

 heat lingering from an initial molten state, a resemblance increased 

 by the added veil of new heat of radio-active origin mantling the 

 ancient stores. 



