R. A. Daly — Abyssal Igneous Injection. 213 



It is manifestly impossible to test these conceptions quanti- 

 tatively, thus comparing their calculated effects with the 

 known needs of the orogenic problem. Yet they are worthy 

 of attention as valuable adjuncts to the thermal-contraction 

 theoiw of mountains. " It may in the end turn out that moun- 

 tains are the result of a tolerably complicated series of causa- 

 tions, in which secular refrigeration of the earth, the transfer 

 of weight by the operations of erosion and deposition, and the 

 subterranean migrations of matter, all take a part."* The 

 energy for crustal deformation may thus lie in the combination 

 of two immense forces — the force involved in massive read- 

 justments within an entire cooling planet, and the force 

 involved in the molecular readjustments among its recrystalliz- 

 ing ultimate particles. Other causes, such as the torsional 

 shears expected in a planet suffering a progressive change in its 

 rate of rotation, may further supplement those causes which 

 have here been briefly noted as seemingly the most important. 



Renewed abyssal injection following monntain-building : 

 development of batholiths. — The extent to which shortening of 

 the transverse axes of the world's mountain ranges has occurred 

 shows that each orogenic revolution has been accompanied by 

 a wholesale shearing of the shell of compression over the shell 

 of tension. The surface of shear is probably not far from the 

 level of no strain. 



One effect of the shearing, faulting and crumpling may be 

 to squeeze small bodies of magma up into the upper shell. 

 But the grandest results of igneous intrusion would be felt in 

 the shell of tension. The instant that the two shells are 

 sheared asunder, the tensions that have been accumulated 

 because of the solid continuity of the two shells, and are still 

 residual after the preceding injection of magma, are relieved. 

 The shell of tension is henceforth free to contract on itself. 

 A fluid dike now injected into this shell or a dike injected 

 previous to the shearing but still fluid, would tend, according 

 to the process already described, and especially because of the 

 energetic, spontaneous retreat of the country-rock on either 

 side, to enlarge itself. Opposed to the active retreat and 

 enforced creep of the solid rock of the shell away from the 

 middle plane of the dike, and thus to the ready contraction of 

 the shell, is the friction developed at the surface of shear. 

 Since the shear is directed tangentially with respect to the 

 curve of the earth, the strength of the friction is measured 

 directly by the weight of the shell above the shear-surface. 

 At the upper extremity of a dike which reaches exactly to the 

 shear-surface, the hydrostatic pressure exerted on the dike-wall 

 is somewhat greater than the weight of the shell above the 

 shear-surface. The magma has, in addition, the live energy 



*N. S. Shaler, op. cit., p. 281. 



