123

geotherms into the sinking mass of sediment and the consequent increase of temperature of the earth-crust beneath.  It will be understood that as these isogeotherms, or levels at which the temperature is the same, lie at a uniform distance from the surface all over the Earth, unless where special variations of conductivity may disturb them, the introduction of material pressed downwards from above must result in these materials partaking of the temperature proper to the depth to which they are depressed.  In other words the geotherms rise into the sinking sediments, always, however, preserving their former average distance from the surface.  The argument is that as this process undoubtedly involves the heating up of that portion of the crust which the sediments have displaced downwards, the result must be a local enfeeblement of the crust, and hence these areas become those of least resistance to the stresses in the crust.

When this theory is examined closely, we see that it only amounts to saying that the bedded rocks, which have taken the place of the igneous materials beneath, as a part of the rigid crust of the Earth, must be less able to withstand compressive stress than the average crust.  For there has been no absolute rise of the geotherms, the thermal conductivities of both classes of materials differing but little.  Sedimentary rock has merely taken the place of average crust-rock, and is subjected to the same average temperature and pressure prevailing in the surrounding crust.  But are there any grounds for the

124

assumption that the compressive resistance of a complex of sedimentary rocks is inferior to one of igneous materials?  The metamorphosed siliceous sediments are among the strongest rocks known as regards resistance to compressive stress; and if limestones have indeed plastic qualities, it must be remembered that their average amount is only some 5 per cent. of the whole.  Again, so far as rise of temperature in the upper crust may affect the question, a temperature which will soften an average igneous rock will not soften a sedimentary rock, for the reason that the effect of solvent denudation has been to remove those alkaline silicates which confer fusibility.

When, however, we take into account the radioactive content of the sediments the matter assumes a different aspect.

The facts as to the general distribution of radioactive substances at the surface, and in rocks which have come from considerable depths in the crust, lead us to regard as certain the widespread existence of heat-producing radioactive elements in the exterior crust of the Earth.  We find, indeed, in this fact an explanation—­at least in part—­of the outflow of heat continually taking place at the surface as revealed by the rising temperature inwards.  And we conclude that there must be a thickness of crust amounting to some miles, containing the radioactive elements.