[1] The calorie referred to is the quantity of heat required to
heat one gram of water, _i.e._ one cubic centimetre of
water—through one degree centigrade.
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It is therefore highly probable that the value found for the mean
radioactivity of acid and basic rocks, or that found for
intermediate rocks, truly represents the radioactive state of the
crust to a considerable depth. But it is easy to show that we
cannot with confidence speak of the thickness of this crust as
determinable by equating the heat outflow at the surface with the
heat production of this average rock.
This appears in the failure of a radioactive layer, taken at a
thickness of about 19-kilometres, to account for the deep-seated
high temperatures which we find to be indicated by volcanic
phenomena at many places on the surface. It is not hard to show
that the 19-kilometre layer would account for a temperature no
higher than about 270° >C. at its base.
It is true that this will be augmented beneath the sedimentary
deposits as we shall presently see; and that it is just in
association with these deposits that deep-seated temperatures are
most in evidence at the surface; but still the result that the
maximum temperature beneath the crust in general attains a value
no higher than 270° C. is hardly tenable. We conclude, then, that
some other source of heat exists beneath. This may be radioactive
in origin and may be easily accounted for if the radioactive
materials are more sparsely distributed at the base of the upper
crust. Or, again, the heat may be primeval or original heat,
still escaping from a cooling world. For our present purpose it
does not much matter which view
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we adopt. But we must recognise that the calculated depth of 19
kilometres of crust, possessing the average radioactivity of the
surface, is excessive; for, in fact, we are compelled by the
facts to recognise that some other source of heat exists
beneath.
If the observed surface gradient of temperature persisted
uniformly downwards, at some 35 kilometres beneath the surface
there would exist temperatures (of about 1000° C.) adequate to
soften basic rocks. It is probable, however, that the gradient
diminishes downwards, and that the level at which such
temperatures exist lies rather deeper than this. It is,
doubtless, somewhat variable according to local conditions; nor
can we at all approximate closely to an estimate of the depth at
which the fusion temperatures will be reached, for, in fact, the
existence of the radioactive layer very much complicates our
estimates. In what follows we assume the depth of softening to
lie at about 40 kilometres beneath the surface of the normal
crust; that is 25 miles down. It is to be observed that Prestwich
and other eminent geologists, from a study of the facts of
crust-folding, etc., have arrived at similar estimates.[1] As a
further assumption we are probably not far wrong if we assign to
the radioactive part of this crust a thickness of about 10 or 12
kilometres; _i.e._ six or seven miles. This is necessarily a
rough approximation only; but the conclusions at which
[1] Prestwich, _Proc. Royal Soc._, xii., p. 158 _et seq._
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