3806 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
Im 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 in- 
compatibility of this hypothesis with the fact of some of the liquid 
interior of the earth rising through the fissures toward 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 vulcanologist, but is contrary to the incon- 
trovertible evidence of tangential compression, as seen in the plica- 
tions and overthrusts 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 Reade and C. Davison? is due the credit of 
making an analytical study of the functions of different 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 contracting, 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 downward. Were the shells of 
cooling and contraction of great tensile strength, such as the experi- 
2@©. Davison: “On the Distribution of Strain in the Earth’s Crust result- 
ing from Secular Cooling, ete.’ (Phil. Trans. R. S., 1887, vol. 178) ; “ 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 re- 
sulting from Secular Cooling” (Phil. Mag., Feb., 1896) ; “On Secular Straining 
of the Earth” (Geol. Mag., May, 1889, Dec. III, vol. 6, No. 299, p. 220). 
T. Mellard Reade: ‘‘ The Origin of Mountain Ranges,” 1886. Seealso H. J. John- 
ston-Lavis: ‘‘ The Extension of the Mellard Reade and C. Davison Theory of 
Secular Straining of the Harth to the Explanation of the Deep Phenomena of 
Voleanic Action ’”’ (Geol. Mag., June, 1890, Dee. III, vol. 7, pp. 246-249). 
