210 



R. A. Daly — Abyssal Igneous Injection. 



accumulates to the depth of many thousands of feet in a geo- 

 synclinal, the material of the original shell of compression is 

 softened by the rising of the isogeotherms, while the strength 

 of the new shell of compression occupied by the sediments is 

 low because of the poor consolidation of this new formation. 

 For a double reason, therefore, a broad zone of weakness in 

 the shell of compression is developed over the zone of igneous 

 injection. Sooner or later the secular accumulation of com- 

 pressive stresses will express itself in the orogenic collapse of 

 the shell ; the building of an alpine mountain range is begun. 

 The most serious objection to the contraction theory — that 

 the earth's amount of contraction is insufficient for the orogenic 



G E S _YN.C_.L-, 



mm 



Fig. 2. — Diagrammatic cross-section showing the relation of a geosynclinal 

 to a zone of abyssal injection. C (vertical lines) — shell of tangential com- 

 pression ; T (blank) — shell of tangential tension ; S (stippled) — substratum. 

 The smaller arrows show the direction of compressive stresses ; the larger 

 arrows show direction of rock-creep in the shell of tension. The original 

 position of the surface before the geosynclinal down-warping is shown by 

 the dotted line. Some of the injected bodies of magma are represented as 

 of batholithic size ; the same amount of rock -creep in the shell of tension 

 and of down-warping of the surface would have been produced by the 

 injection of smaller bodies, more numerous and also closely spaced in the 

 zone of injection. Scale : about 65 miles to one inch ; the curvature of the 

 earth's surface is exaggerated about ten times. 



work actually done through geological time — has been founded 

 on mathematical deductions. As with the so-called demon- 

 stration of the earth's extreme rigidity, the deductions are no 

 stronger than the assumptions as to the interior economy of 

 the globe. If the earth's vast nucleus is gaseous, as seems so 

 highly probable, the larger part of the earth may be cooling 

 and contracting according to different laws from those hitherto 

 accepted as a basis for calculation.* For example, the con- 

 traction of the nucleus may follow a modified "law of Charles"; 

 secondly, we should expect that, with cooling, there will be, 

 within the globe, the liquefaction of gas as well as the solidifi- 

 cation of liquid— both changes of state possibly being accom- 

 panied with special diminutions of volume. The phenomenon 

 of earth-contraction may thus be much more complicated than 

 it has been assumed to be, but the added complications would 

 probably be favorable to the contraction theory of crustal 

 deformation. 



* Cf. S. Arrhenius, Geol. Foren. Stockholm Forhandlingar, xxii, p. 395. 



