JR. A. Daly — Abyssal Igneous Injection. 



207 



sion operating wherever a line of diminished pressure is devel- 

 oped in the viscous layer. 



It is also manifest that, if torsional or other shear is accom- 

 panied by vertical faulting, the abyssal injection will be still 

 further facilitated. 



Relief of tensions through abyssal injection. — Whether the 

 foregoing hypothesis be correct in details or not, there is no 

 doubt that abyssally-injected dikes have actually been fed from 

 the gabbroid substratum upward to the vents of fissure erup- 

 tion. Granting, secondly, that there are cooling cracks and 

 considerable unrelieved tensions in the crust, as already 



Surface 



SHELL OF 

 COMPRESSION 



o f 



Approximate lowet 



TENSION 



limit of shell of fractiil 



Ql-P- 



+ + + + + + + + + + + + + SUBSTRATUM + + + l-+y + <- + + + + + + + + 



t H t H f t H t +.H H H H f H f t K + H +-H H H f H t 



described, consequences of fundamental importance seem to 

 be deducible. 



On account of the strong compression at the earth's surface 

 the magma of the abyssally-injected dikes will not in most 

 cases reach the surface. The act of injection produces a great 

 change in the conditions of equilibrium in the shell of tension 

 and therewith in the whole crust. 



Let figure 1 represent a sectional view of the system after 

 injection, the earth's curvature being neglected and the dike 

 being shown in cross-section. The level of no strain is repre- 

 sented as about five miles below the surface — a depth some- 

 what greater than the maximum calculated by Fisher. — The 

 principle of the following argument is not affected if the depth 

 should be a fraction of one mile or as much as six or possibly 

 more miles. 



" A " is a particle of the crust within the solid shell of ten- 

 sion. In the stretching of the shell such a particle must move 

 not only radially toward the earth's center but tangentially as 

 well. If the shell is homogeneous, the weight of the over- 

 lying crust will tend to shear the particle indifferently toward 



