196 JR. A. Daly — Abyssal Igneous Injection. 



tions which are more or less reasonable provided the fact of 

 complete solidity is established. Their estimates for the depth 

 of the zero-strain level vary from 2 miles to Y'79 miles. * 

 With analogous assumptions Fisher has calculated that there 

 will similarly be a level of zero-strain in a crust overlying 

 the fluid substratum of a globe solidifying from the circum- 

 ference inwards. He found that "if the time elapsed since a 

 crust began to be formed has been 100 million years, the 

 depth of the level of no strain at the present time will be 

 about four miles."*!* In any case the depth increases very 

 slowly with the time elapsed since the crust first formed. 



Rudski has pointed out that, if the earth's initial temper- 

 ature were not uniform, the level of no strain would, in a 

 given time, be deeper than by the amount calculated on the 

 assumptions of Davison.;); It is, in truth, probable that the 

 initial temperature increased downwards. We shall see that 

 there is a grave reason for doubting the conclusion of Kelvin 

 that an initial uniform temperature was secured through the 

 foundering of early crusts. The suggestion of LeConte that it 

 might be secured through the operation of convection currents 

 is not acceptable to those who hold the very probable view 

 that the earth's internal density increases downward, not only 

 because of increasing pressure but because of differences in 

 chemical composition as well.§ 



All of these calculations have been made on the supposition 

 that the thermometric conductivity of the material of the earth is 

 a constant quantity. It is, however, most probable that this 

 conductivity decreases with rise of temperature and very 

 greatly increases on the passage of liquid magma into solid 

 rock. Forbes showed that the " calorimetric " conductivity of 

 iron decreases with rise of temperature, as illustrated in the 

 following table, which is an abridged form of his experimental 

 results : || 



* C. Davison, Philosophical Transactions, Eoyal Society of London, vol. 

 clxxviiiA, p. 231, 1887; G. H; Darwin, ibid., p. 242; C. Davison, Proceed- 

 ings of the Eoyal Society of London, vol. lv, p. 141, 1894. Cf. M. Eeade, 

 Origin Of Mountain Eanges, London, 1886, p. 121. 



fO. Fisher, Physics of the Earth's Crust, London, 2d ed. 1891, appendix, 

 p. 45. 



JM. M. P. Eudski, Philosophical Magazine, vol. xxxiv, p. 299, 1892. 



§Cf. J. LeConte, American Geologist, vol. iv, p. 43, 1889. It may be 

 noted that, in the above-mentioned calculations, no account has been taken 

 of the special and important contraction characterizing the passage of lava 

 from the liquid to the solid state, nor, except in the case of Fisher's estimates, 

 for the fact that, with a given fall of temperature, liquid lava (diabase) con- 

 tracts about twice as much as solid lava (Barus, Bull. U. S. Geological Sur- 

 vey, No. 103, 1893). 



| J. D. Forbes, Trans. Eoy. Soc. Edinburgh, xxiv, p. 105, 1867. 



