208 M. A. Daly — Abyssal Igneous Injection. 



m or m' or toward any one of an infinite number of other 

 points lying in the circumference of a horizontal circle circum- 

 scribed about the vertical passing through A and with radius 

 Am. The shear-movement of particle A is, however, strictly 

 controlled in direction so soon as a liquid dike is injected. At 

 the level of A the point O in the wall of such a dike bears a 

 combined hydrostatic and elastic pressure from the magma. 

 The former pressure is sensibly equal to the weight of the 

 column of rock Ax ; the maximum elastic pressure equals the 

 weight of the column Ay. The total of these pressures, repre- 

 sented by the line On, is equal to the oppositely directed force 

 O'n' on the other wall of the dike. On is not only a positive 

 force compressing the matter between O and A; it is also, and 

 yet more significantly, a directive force which determines the 

 direction in which particle A must move as it is affected by 

 the tensional pull of secular cooling and by shear during the 

 compressive extension (stretching) of the shell of tension. As 

 long as the dike remains fluid, particle A will move in the 

 direction of the arrow Am' . The condensation of matter, 

 which, before the dike-injection, had been only potential 

 (being due to the accumulation of tensions and cracks in the 

 shell), now becomes actual. As particle A is forced toward m', 

 a neighboring particle, A 1 on the same level and to the right 

 hand of A, is similarly brought under pressure and moved in 

 the direction of the arrow Am' . A 1 communicates its motion 

 to A 2 and so on. The pressure at O is thus felt within the 

 shell as far away from the dike as the relief of the accumu- 

 lated tension and the closing of cooling cracks can take place. 



The movement of particles A, A,, A^ etc., is analogous to 

 the work of a railway engine pushing down a train of cars 

 which had been standing on a grade with each coupling pin at 

 full length because of the grade. Buffer meets buffer, com- 

 municating the pressure of the engine. If the train had been 

 nicely poised, just ready to move before the pressure was 

 applied, and if the grade were indefinitely long, a small pres- 

 sure would set in motion a train of indefinite length. The 

 analogy is not perfect since the creep of the particles in the 

 shell of tension is not free but is controlled by internal fric- 

 tion and by the strong adhesion between the shells of com- 

 pression and tension. Nevertheless, it is not difficult to believe 

 that lateral creep would be set up at a distance perhaps several 

 times the thickness of the whole crust. 



Since the conditions are precisely the same for particles B, 

 B x (to the left of B), B 2 , etc., there will be similar creep on 

 the side of the dike opposite to A in the direction of the 

 arrow O'n'. The dike is thereby widened. The correlative 

 injection of new fluid magma makes this new system of 



