is required merely to explain the Tertiary folding. For more ancient periods, when tectonic movements were much more universally in operation, still higher values will be required. This, however, is in direct contradiction to the results of theoretical physics, for Lord Kelvin calculated from the present weak flow of heat from the centre of the earth to the surface that a temperature of the body of the earth so much higher in former periods is quite out of the question. Rudzki[1] has, of course, drawn attention to the fact that in Lord Kelvin’s calculation no allowance is made for the work done in compression under the force of gravitation, in consequence of which, in spite of the loss of heat, the temperature remains nearly constant, and contraction is almost all that occurs; but nevertheless Rudzki hastens to add the conjecture that the above-quoted coefficients of expansion might probably be considerably decreased on account of the high pressure existing in the earth, so that Lord Kelvin’s calculations would again be correct. To sum up, it may be said that theoretical geophysics on this aspect of the subject does not yet lend itself to definite conclusions. The researches on radium appear to offer more unequivocal results. Great quantities of heat are generated through the automatic disintegration of radium. This element in any case exists in all rocks in such large traces, according to Joly’s determinations, that if the same radium content persisted to the centre of the earth,[2] the constant conduction of heat from the earth’s interior, which can be verified by measurement of the increase of temperature with depth in mines, would be more than compensated.
Whether we must therefore assume, as Strutt believes
