201 R,. A. Daly — Abyssal Igneous Injection. 



flow. The flow of marble under confinement has been pro- 

 duced under relatively low pressures, but this is a special phe- 

 nomenon, the result of movement on gliding planes. A pen- 

 knife and a few pounds of pressure will cause " flow " in a 

 crystal of calcite. It is safe to say that similar conditions are 

 not found in the average rock of the crust ; if it flows at all the 

 mechanism of the flow must be something entirely different. 



Deformation within the shell of tension is not to be esti- 

 mated simply by the ultimate strength of surface rock deformed 

 in the laboratory. The experiments of Spring, Hallock and 

 others show that the rigidity of a solid increases with pres- 

 sures ranging up to those about twice that borne by our sub- 

 stratum.* This experimental law strengthens the belief that 

 cavities may remain open in the shell of tension. On the 

 other hand, the downward increase of temperature tends to 

 lower the internal friction and thus to promote the closing of 

 cavities. The pressure-gradient (1 atmosphere to about 3*7 

 meters of descent) is, however, steeper than the temperature 

 gradient (1° C. to about 30 meters of descent) and it may well 

 be that rigidity actually increases through the shell of tension 

 down to its bottom layer, where, on account of the high tem- 

 perature, the change of state, from solid to liquid, is approached. 



A further indication that cavities may remain open in the 

 shell of tension is indirect but none the less noteworthy. 

 According to the assumption generally held by those adopting 

 the contraction theory of mountain-building, the shell of tan- 

 gential compression, free of load and unconfined as it is along 

 its upper surface, can nevertheless for long periods of time 

 endure without deformation a compressive stress perhaps sev- 

 eral times greater than the weight of five miles of rock. It is 

 the release of this pressure (which was not relieved by simple 

 radial flow and thickening of the shell) that has led to the 

 paroxysmal growth of a mountain-range. If the outer shell 

 can long withstand such pressures, it is reasonable to believe 

 that the material of most of the shell of tension is not perfectly 

 plastic under the weight of overlying crust, — a pressure which 

 is great but, in general, is only a fraction of the accumulated 

 tangential stress of compression. 



The same argument seems to apply also to the conceivable 

 closing of cavities through the expansion of the compressed 

 wall-rocks which tend to expand elastically into the opening 

 vertical crack. For the reasons already outlined, this expan- 

 sion must, under the conditions, take place, through most of 

 the shell of tension, by shearing of mass against mass rather 

 than by molecular flow. The induced partial closing of the 

 cavity would, here again, tend to prevent further shearing and 



*For references see review by C. F. Tolman, Jr., Journal of Geology, vol. 

 vi, p. 323, 1898. 



