B. A. Daly — Abyssal Igneous Injection. 209 



motions self -perpetuating until the attainable relief of tensions 

 and closure of cracks is accomplished. 



Thereafter, two possibilities are open. The now much 

 widened dike may have lost sufficient heat to solidify. The 

 system of directed creeps or lateral movements will then be 

 exchanged for an undirected compressive extension similar to 

 that which prevailed before the injection. Or, if the dike 

 remains fluid, it will cause an indefinite continuance of lateral 

 creep keeping pace with the differential cooling contractions 

 in the shell of tension. In the former case, the injection of a 

 second and of yet later dikes is possible, and their net effects, 

 provided these dikes are elongated in the same general earth- 

 zone," are additive to those of the first dike. Tidal or other 

 torsion may locate such a zone of special igneous injection. 



Down-warping of the surface as a result of abyssal injec- 

 tion. — We have seen that lateral creep will be fastest some- 

 where near the middle level of the shell of tension, because it 

 is there that the defect of condensation of matter, shown in 

 cooling crack and in residual tension, is'at a maximum. The 

 ensuing condensation of matter in the shell is at a maximum in 

 the immediate vicinity of the zone of injection and gradually 

 decreases to each side of the zone. Since the two shells are 

 still solidly knit together, the enforced creep of matter to right 

 and left of the great dikes involves a strong downward pull 

 exerted on the shell of compression. A down- warp of the 

 earth's surface is thus established. The initial down-warp is 

 of length, breadth and depth dependent on the magnitude of 

 the injected body or bodies. Where the injection is on a large 

 scale the down-warp may be of geosynclinal dimensions. 



The down-warping implies, however, that the former nice 

 balance of stresses in the zone of compression is destroyed. 

 Those stresses will henceforth tend directly to increase the 

 down-warp. Sedimentation within the dowmwarp increases 

 the weight on the creeping material of the shell of tension, 

 which is also now beginning to feel a small downward pres- 

 sure, a component of the total thrust of the now bent shell of 

 compression. The down-warping of the surface may thus 

 gradually increase even after all magmatic injections in the 

 zone of tension have frozen solid. 



The conditions for mountain-building. — The shell of com- 

 pression is now weakened, as experimentally illustrated by 

 Willis in his memoir on mountain-building.f The weakening 

 is most felt in the two lines where the down- warped surface 

 parts from the spheroidal curve of the earth. If sediment 



* Throughout this paper the word ''zone" is used in its proper mathe- 

 matical sense of a "portion of the surface of a sphere included between 

 two parallel planes." — As prevailingly defined, the " zone of fracture" in 

 the earth may here be profitably called the " shell of fracture." 



f B. Willis, 13th Annual Report, United States Geological Survey, p. 217, 

 1893. 



