DALY. — THE NATURE OF VOLCANIC ACTION. 55 



Honolulu in 1887. Each of these books first became accessible to the 

 writer in I'JOy. It is certainly a pity that the second part of Green's 

 work is not more generally known. The book is almost as remarkable 

 a contribution to the philo.sophy of vulcanism as Part I, on the Tetra- 

 hedral Theory of the Earth, is important in cosmogonic philosophy. 

 With the advance of geologic exploration, the arguments for a univer- 

 sal basaltic substratum are to-day more convincing than ever. For the 

 sake of brevity these arguments will not here be fully presented, though 

 some of them are mentioned in the sequel. 



It will further be assumed that the known temperature gradient may 

 roughly indicate the depth (estimated at 40 kilometers) at which the 

 basaltic substratum is actually or potentially fluid. Many writers have 

 independently arrived at nearly ec^uivalent estimates for this depth. 

 At such depth the internal friction may be so increased by the pressure 

 that the substratum almost perfectly resists deformation by the quickly 

 acting tidal forces.^^ This assumption in no way excludes the possi- 

 bility that the substratum basalt is a true, non-crystalline fluid, and 

 that a powerful force, slowly applied (on the principle of stress ditt'er- 

 ences), could force this highly viscous fluid into the solid crust. As 

 the fluid rises, its viscosity must fall directly with the lessened ])ressure, 

 from a possible initial value of perhaps many millions of millions of 

 times that of water to that of less than fifty times the viscosity of 

 water, when the fluid reaches the earth's surface.*' 



SoAiE Direct Consequences of Abyssal Injection. 



The estimate of 40 kilometers for the average depth of the surface 

 of the substratum may be wide of the mark, but it will serve as 

 the numerical basis for a statement of certain immediate effects of 

 injection. 13 



" Ilorglotz h:i.s rofently conclu(lo(l that a planot stratified according to den- 

 sity must resi.st tidal deformation Ix-tter than would the homoRene<ius planet 

 a.s.sumed by lA)rd Kelvin. G. Ilerglotz, Zeit. fur Math. u. I'liy.-*. 62, 27.1 

 (190.5). 



" Reeker computes the viseoaity of a Hawaiian lava flow (whicli w.a.s by 

 no means the most fluid already ob.served) as no more than sixty times that 

 of water. G. V. Becker, Amer. Jour, of Science, 3, 29 (1X97). 



" Perhaps the surface of the substratum is !us deep within the earth .as the 

 limiting depth of i.sostatic compensation. In the .second of his notable 

 memoirs concerning the figure of the earth and isostiusy, Havford .states that 

 thf most probalde value of this limiting depfli, for the I'nited States and 

 a<ijarent area.s, is 122 kilometers, if it be jussumcd that the isostatic compen- 

 sation is uniformly di.stributed with respect to dejjth. See |)age 77 of hia 

 "Sui)plementary Investigation in 1909 of the Figure of the Earth and Isoa- 



