282 JR. A. Daly — Mechanics of Igneous Intrusion. 



pletely."* Whether solid or molten when sheared or pulled 

 out, such blocks could not sink in the magmas because of their 

 thick-pasty condition. 



Overhead stoping in magmas of high fluidity. — If then, a 

 visible contact zone shows that its magma could, in the 

 enfeebled condition just prior to solidification, rift off blocks 

 in great number from the invaded formation, it is clear that 

 the same magma could carry on the destructive work much 

 more rapidly when highly fluid. Not only would the many, 

 often intersecting, apophyses more readily force their way into 

 the country rock ; the certainty of loss to the walls and roof 

 would be increased because of direct gravitative pull. Blocks 

 having insufficient attachment to the roof because of joints or 

 other planes of weakness in the superstructure, would sink in 

 the magma less dense than themselves. 



Another cause of the mechanical destruction of the vault, a 

 cause, perhaps much more important than the two cited, may 

 be found in the special conditions of strain existing at molar 

 contacts. The temperature of the invaded rock is raised by 

 the adjacent magma many hundred degrees Centigrade above 

 the temperature the rock may be assumed to have had before 

 the intrusion began. As much as two per cent of volumetric 

 increase could thus be produced in the solid rock close to 

 the magma. Farther away, though still near the contact, the 

 elevation of temperature and corresponding expansion in the 

 country-rock would be of a much lower order. It is evident 

 that enormous strains would be set up in the relatively thin 

 shell of the vault bounded by the molar contact. The strains 

 would be comparable to those observed in surface cliffs and 

 quarries exposed to rapid but small changes of temperature, 

 but on a much greater scale. The complex stresses induced 

 might conceivably result in the extensive shattering and exfolia- 

 tion of the country-rock. f 



In the operation of all these various causes, it is highly 

 probable that, during most of its history, a molten plutonic 

 magma has a much greater power of rifting masses from its 

 walls and roof than in the short closing stage of its career as 

 such before crystallization is completed. 



The opening of the magma chamber. — By the integration 

 of relatively small effects in stoping, the chamber or space 

 opened for the continued intrusion of a magma may be 

 indefinitely enlarged, so long as heat and thickness of crust 

 sufficient are supplied by the conditions of nature. Its form 

 would be that of a downwardly enlarging compartment within 

 the invaded formation, though a pipe-like chamber could also 



* Ann. Eep. Geol. and Nat. Hist. Surv., Canada, 1887, Pt. F, pp. 131-2-3-8, 

 etc. 



f Of. Reade's experiments, op. cit. pp. 17-28. 



