434 Dr. Johnston-Lavk — -Mechanism of Volcanic Action. 



In the old theory of the earth-crust crumpling over a contracting 

 and cooling nucleus, fluid, or partially so, it always appeared to me to 

 be inexplicable how fluid matter could be squeezed out, or why the 

 water on the surface of the earth did not rush down to fill up the 

 vacancy that the contracting interior tended to produce between itself 

 and the arch of the crust. This perhaps is expressing the facts 

 in simple commonplace terms, but is sufficient to illustrate the 

 incompatibility of this hypothesis with the fact of some of the liquid 

 interior of the earth rising through the fissures towards the surface 

 and being squeezed out by the contracting crust. 



The hypothesis that tangential thrust did not exist, but that the 

 earth-crust was shrinking on an entirely or partially fluid nucleus, would 

 have satisfied the vuleanologist, but is contrary to the incontrovertible 

 evidence of tangential compression, as seen in the plications and over- 

 thrusts existing upon the entire surface of the globe, or at least that 

 part above sea-level. This hypothesis was based upon the conception 

 that the earth's crust was acting as a single unit. 



To Messrs. Mellard E.eade and C. Davison^ is due the credit of 

 making an analytical study of the functions of difl'erent parts of 

 the earth's crust. That work demonstrated that theoretically we 

 can divide the cooling surface of the earth into a series of 

 shells. The outer shells that have reached approximately the 

 mean atmospheric temperature will, of course, have stopped con- 

 tracting, whereas the shells nearest to the heated nucleus will be 

 those losing their heat most rapidly, and therefore undergoing 

 greatest contraction. This contraction must inevitably cause crowding, 

 crushing, and crumbling of those shells that are nearer the surface, 

 just as a stretched sheet of rubber coated with a layer of stiff clay 

 would do when allowed to contract. 



Somewhere between the surface shells of compression and the 

 deepest shells of greatest cooling and contraction there will be a shell 

 in a state of equilibrium, which the authors call the zone of no 

 contraction. This zone, which was originally quite at the surface of 

 our globe, tends to sink lower and lower as the general refrigeration 

 or isotherms of our planet proceed downwards. Were the shells of 

 cooling and contraction of great tensile strength, such as the 

 experimental sheet of rubber, already referred to, we can quite 

 understand how any fluid in the earth's interior would tend to be 

 squeezed out, but are met by two difficulties — (1) is there any fluid 

 in the earth's interior? and (2) is the tensile strength of the contractile 

 shells sufficient to have a squeezing power ? 



Three classes of views have been held as to the constitution of our 



1 C. Danson : " Oa the Distributiou of Strain in the Earth's Crust resulting 

 from Secular Cooling, etc." (Phil. Trans. E.S., 1887, vol. clxxviii) ; "Note on 

 the Relation between the Size of a Planet and the Rate of Mountain Building on 

 its Surface" (Phil. Mag., Nov., 1887) ; " On the Straining of the Earth resulting 

 from Secular Cooling" (Phil. Mag., Feb., 1896) ; "On Secular Straining of the 

 Earth" (Geol. Mag., May, 1889, Dec. Ill, Vol. VI, No. 299, p. 220). 

 T. Mellard Reade : The Oriffin of Mountain Ranges, 1886. See also H. J. Johnston- 

 La vis: "The Extension of the Mellard Reade and C. Davison Theory of Secular 

 Straining of the Earth to the explanation of the Deep Phenomena of Volcanic 

 Action" (Geol. Mag., June, 1890, Dec. Ill, Vol. VII, pp. 246-9). 



