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 escape of about 0.0014 calorie[1] per square metre of the Earth’s surface per second (assuming the rise of temperature downwards, i.e. the “gradient” of temperature, to be one degree centigrade in 35 metres) the downward extension of such rocks might, prima facie, be as much as 19 kilometres.

About this calculation we have to observe that we assume the average radioactivity of the materials with which we have dealt at the surface to extend uniformly all the way down, i.e. that our experiments reveal the average radioactivity of a radioactive crust.  There is much to be said for this assumption.  The rocks which enter into the measurements come from all depths of the crust.  It is highly probable that the less silicious, i.e. the more basic, rocks, mainly come from considerable depths; the more acid or silica-rich rocks, from higher levels in the crust.  The radioactivity determined as the mean of the values for these two classes of rock closely agrees with that found for intermediate rocks, or rocks containing an intermediate amount of silica.  Clarke contends that this last class of material probably represents the average composition of the Earth’s crust so far as it has been explored by us.

[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 deg. >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 deg.  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