B. A. Daly — Abyssal Igneous Injection. 215 



compression of the superficial shell is relieved. Otherwise 

 stoping is impossible, for a large magma chamber could not 

 remain open in the zone of unrelieved compression. Granites 

 and allied rocks are, by this hypothesis, primarily mountain- 

 rocks. That this is the fact hardly needs statement. 



Yulcanism as a result of mountain-building. — By the 

 inevitable settling-down and block-faulting which follow the 

 orogenic paroxysm, "both primary basaltic lava and secondary 

 lavas of indefinite variety may be squeezed out to the surface. 

 Yolcanic activity is not, by the hypothesis, necessarily confined 

 to zones of intense mountain-building, but should be specially 

 developed in those zones. The volcanic problem and the 

 orogenic problem are in general both related to the same 

 necessity of understanding the mechanical rearrangements when, 

 for any reason, fluid material from the substratum is injected 

 into the shell of tension. 



Summary. 



Postulates. — The assumptions on which the foregoing 

 hypothesis has been based are the following : 



a. A cooling earth superficially composed of a relatively 

 thin crust overlying a fluid gabbroid substratum of unknown 

 thickness. 



b. The substratum so much compressed by the weight of the 

 crust as to be probably able to float the crust. 



c. Through differential cooling contraction the development 

 of a level of no strain in the crust not far from the bottom sur- 

 face of the shell of rock-fracture. 



d. The accumulation of pressure in the shell of compression 

 and the simultaneous accumulation of cooling cracks and of some 

 of the powerful tension unrelieved in the shell below the level 

 of zero-strain. 



e. A steady or recurrent dislocation of the shell of tension 

 permitting of the forceful injection of the fluid substratum to 

 which even the viscous layer of the shell acts as a relatively 

 solid mass at the moment of dislocation. This dislocation has 

 been referred to the tidal torsion of the earth's crust, but sub- 

 equatorial torsion on the tetrahedral theory of the earth, or 

 crustal deformation due to the play of other cosmical forces 

 or of forces induced by the heterogeneity of the crust, may simi- 

 larly cause dislocation in the shell of tension. 



Conclusions. — 1. The abyssal injection involves condensation 

 of the matter in the shell of tension. Cracks are closed and much 

 of the accumulated tension is relieved by an enforced creep of 

 matter away from the injected body. So long as the body re- 

 mains fluid the stretching of this shell due to continued cooling 

 of the earth is accomplished by creep of matter in the same 



