212 It. A. Daly — Abyssal Igneous Injection. 



enforced and illustrated by Shaler* and must be regarded as 

 embodying a true condition of crustal deformation. 



b. Still more important is the consideration that the shell of 

 compression is the home of those metasomatic changes in 

 rocks that lead to expansion of volume. One of the most strik- 

 ing results of Yan Hise's researches in metamorphism is his 

 conclusion that in this shell the average effect of hydration, 

 carbonation and oxidation is to cause such expansion on a sur- 

 prisingly large scale. f For example, if a gabbro were com- 

 pletely altered according to the normal reactions in the " kata- 

 morphic zone " (which, in position and depth, is very nearly 

 identical with our shell of compression), there would be a 

 volumetric increment of at least 25 per cent. If the entire 

 shell were gabbroid, and if but 4 per cent of its substance had, 

 in post-Archean time, been similarly hydrated and carbon- 

 atized, the volumetric increase would be about sufficient to 

 explain all of the observed overthru sting and overfolding of 

 post-Archean mountain-ranges. The rocks of the continental 

 plateaus are, however, largely composed of quartz and ortho- 

 clase, two minerals which do not show volumetric expansion 

 in their alteration. The part of the shell underlying the deep 

 ocean-basins is of unknown composition, but pendulum obser- 

 vations and other general considerations suggest that this 

 greater part of the shell is basaltic, and has, in general, never 

 been exposed to subaerial erosion. Under the deep seas especi- 

 ally the maximum amount of metasomatic expansion and the 

 maximum accumulation of corresponding compressive stress 

 might, accordingly, be expected. It is conceivably to this 

 cause that we may refer the fact that the thrust of mountain- 

 building has, throughout the world, been chiefly from the 

 ocean toward the land. In any case the average result of the 

 alteration of rocks, whether by cold descending waters or by 

 hot ascending waters, or by water trapped within the shell, 

 and with all allowance made for solution of mineral matter 

 which is thus removed to the oceans, is to bring about expan- 

 sion of volume within the shell. This expansion must be 

 accompanied by tremendously energetic compressive stresses; 

 the process is homologous to the hydration of a bed of anhy- 

 drite. Since these chemical reactions take place mainly along 

 more or less vertical cracks, fault-planes, joints, etc., the expan- 

 sional force will be chiefly directed in planes parallel to the 

 earth's surface. The relief of the strains through simple 

 vertical expansion is resisted by the strength of the unaltered 

 rock lying between the vertical zones of chemical alteration. 



* N. S. Shaler, Science, xi, p. 280, 1888. 



\ C. E. Van Hise, Treatise on Metamorphism, Monograph xlvii, U. S. 

 Geol. Surv., pp. 631 ff. 



