J?. A. Daly — Abyssal Igneous Injection. 205 



portions of the crack would remain unclosed. It is important 

 to note that the shearing of mass against mass due to expansion 

 along the walls is not additive to the effect of mass-shearing 

 due to the dead weight of crust ; the two kinds of deformation 

 would progress simultaneously, and in proportion as masses 

 moved under the one kind of stress, it would become more 

 difficult for mass-shearing under the other kind of stress to take 

 place. Only at the bottom of the shell of tension where the 

 crust-matter is in the state intermediate between those of plastic 

 solid and viscous fluid, would the cavities be closed entirely. 



This part of the argument may now be summarized. On 

 the whole it seems probable that a percentage of the whole 

 tension developed in the lower shell through secular cooling 

 remains, at any time previous to mountain-building, unrelieved 

 by the stretching or cracking of that shell. At the level of 

 zero-strain (which is above or not far below the bottom of the 

 " shell of rock-fracture") cooling tension is at a minimum and 

 resistance to stretching (shearing) is at a maximum. At the bot- 

 tom of the crust the cooling tension is at a maximum but the 

 resistance to stretching is at a minimum. The accumulation 

 of tension and cooling cracks will therefore be at a maximum 

 at some level near the middle of the shell of tension. The 

 accumulation of compressive strains in the outer shell will be 

 relieved to a certain extent by recrystallization leading to the 

 development of denser minerals in the shell ; but geological 

 observation shows that, in a long period of time, enormous 

 compressive stresses are alway stored until relieved by a more 

 catastrophic process. The accumulation of the tensile stresses 

 in the lower shell will be in some direct proportion to the 

 degree in which relief is withheld in the shell of compression. 

 Beneath a crust so diversely stressed, there is a compressed, 

 elastic fluid which is ready, with relative suddenness and with 

 prodigious force, to inject itself into the shell of tension as 

 soon as there is any local relief of pressure or any breaking of 

 the continuity of the shell. 



The whole system is evidently in unstable equilibrium. If 

 each shell were of uniform thickness and composition, and if 

 there were no external forces acting on the system, it would 

 be difficult to forecast when or where the strains could be 

 relieved. 



Injection of magma into the shell of tension. — But the 

 earth's crust is not perfectly homogeneous ; none of the shells 

 is of perfectly uniform thickness ; and, thirdly, there are other 

 powerful forces acting on the material of the shell of tension 

 besides those leading to stretching during the earth's contrac- 

 tion. Of special importance is the shearing of the whole crust 

 in the torsional deformation incidental to the contraction, or 

 in the torsion due to tidal stress. Slight as may be the effect 



