214 E. A. Daly — Abyssal Igneous Injection. 



of elastic expansion measured by the compression due to the 

 weight of the whole shell of tension. The net effect of these 

 forces is to permit of the contraction of the shell already prone 

 to movement on account of the sudden relief of tension, and to 

 cause a widening- of the dike which may assume batholithic 

 proportions. It is important to note that the recoil within the 

 shell due to the relief of tensions will characterize the whole 

 of the area over which the shells of tension and compression 

 have been sheared apart ; this area may be several thousand 

 miles in diameter. The piling up of the mountain -mass above 

 would also cause an enhanced rapidity or lateral flow in the 

 shell of tension and likewise widen the magmatic chamber. 

 Injection into the mountain-rocks themselves would only be 

 possible where there is local relief of compression in the now 

 heterogeneous, unequally squeezed, and writhing mass. Since, 

 in the nature of the case, compression generally dominates, 

 igneous injection will, in this period, afford but small geologi- 

 cal bodies as constituents of the range. 



At the mountain-roots below the surface of shear there are 

 one or more great bodies of gabbroid magma specially injected 

 as a result of mountain-building. Through the physical and 

 chemical activity of this magma the acid igneous stocks and 

 batholiths so characteristic of mountain-range of the alpine 

 type, may possibly be explained by the assimilation-differentia- 

 tion theory.* The cycle of changes which have affected our ini- 

 tial system (compression-shell, tension-shell and fluid substra- 

 tum) thus began with igneous intrusion and closes with igneous 

 intrusion. On account of the relief of compressive strains in 

 the superficial shell, the latest and probably greatest intrusive 

 bodies are free locally to flux or stope their way well into the 

 shell of fracture. 



With certain assumptions, several authorities have calculated 

 that the level of no strain has always lain at a depth no greater, 

 or but very little greater, than the bottom of the shell in 

 which rocks can readily fracture.f In rising through the shell 

 of tension the gabbroid magma has expanded so much and 

 attained such low viscosity that down-stoping and abyssal 

 assimilation of shattered roof-blocks is now possible. To the 

 differentiation of the compound magma so produced, the 

 granitic batholiths and stocks, many injected laccoliths, " chono- 

 liths " and dikes, as also many lavas more acid than gabbro, 

 have been attributed. There are reasons for believing that 

 magmatic stoping is much more potent than fluxing assimila- 

 tion on main contacts. If granites, etc., are really secondary 

 after the manner indicated, they can only be formed in the 

 shell of fracture and on the large scale where the tangential 



*See E. A. Daly, this Journal, vol. xv, 1903, p. 269; vol. xvi, 1903, p. 107; 

 vol. xx, 1905, p. 185. 



f The limits of the shell of ready fracture, as conceived by Van Hise, are 

 shown in Fig. 1. 



