ment 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

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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.

Some of the most recent measurements of the quantities of radium
and thorium in the rocks of igneous origin--_e.g._ granites,
syenites, diorites, basalts, etc., show that the

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radioactive heat continually given out by such rocks amounts to
about one millionth part of 0.6 calories per second per cubic
metre of average igneous rock. As we have to account for the
es