DALY. — THE NATURE OF VOLCANIC ACTION. 53 



speculatively considered by the writer in a paper published in 1906.* 

 Therein the general result of the work of Reade, Davison, Fisher, and 

 G. H. Darwin, on the stresses produced in the earth through cooling, 

 was assumed as true. A superficial shell of compression is separated 

 from a much thicker underlying shell of tension, by a level of no strain. 

 The tensions of the lower shell and the failure of complete "compres- 

 sive extension " in that shell furnish the conditions favoring magmatic 

 intrusion into the shell. The writer's earlier statement (page 204 of 

 the paper referred to), that laterally-confined rock-matter grows vastly 

 more rigid with increase of pressure, has been recently corroborated in 

 striking fashion by Adams and Bridgman in their respective high-pres- 

 sure experiments. Adams shows that, with lateral support at high 

 pressure, the normally weak mineral, fluorite, became less plastic than 

 hard steel at ordinary pressure.'' P. W. Bridgman (verbal communica- 

 tion) has proved that under external hydrostatic pressure amounting to 

 24,11(10 atmospheres, the cavity of a sealed glass tube is not closed, even 

 after prolonged application of such enormous force. In an actual test, a 

 tube, having undergone a hydrostatic pressure of 24,000 atmospheres 

 — corresponding to the weight of more than eighty kilometers of 

 the earth's crust — for a period of three hours, was removed from the 

 press and was found to have undergone no sensible change in size 

 or form. These experiments seem to sustain the view that at least 

 the upper, cooler part of the shell in which tension is developed on 

 account of the cooling of the earth, is not kept fully and continuously 

 condensed by the weight of the overlying shell. On the contrary, the 

 shell of tension may be capable of considerable condensation, which 

 awaits, as well as permits of, the periodic injection of liquid matter 

 from the substratum. This is also the conclusion of Lane, expressed 

 in his highly suggestive paper on the " Geologic Activity of the Earth's 

 originally absorbed Gases," where he has considered the mechanics of 

 abyssal fissuring.* 



In general, the injections must find difficulty in passing beyond the 

 level of no strain, which is only a few kilometers below the surface of 

 the planet. In the cases where the magma does succeed in penetrating 

 the shell of compression, volcanic action ensues. 



Since the speculation is not vital to the argument of the present 

 paper, it will not be further reviewed. It may be noted that the same 



• R. A. Duly, Al)yMs;il Igneous Injection ;ls aCiiusal Condition and ;id an 

 Effect of Mountain-building. Am(;r. .Jour. Science, 22, l'Jo-21G (,1'JUlj). 

 » r. D. Adam.s, Jour. Gcol., 18, 500 (1910). 

 » A. C. Lane, Bull. Geol. Soc. America, 6, 2G9 (1894). 



