270 Arthur Holmes—Radto-activity. 
represents the distribution of temperature in an earth which has 
cooled down for 1,600 million years from an initial temperature at 
or near the surface of 1,000° C. Curve II] represents the distribution | 
of temperature due to radio-active elements alone, and here ma is 
subtracted from the value at every depth x on the assumption that 
mis 5° per km., in order that the curves may be comparable. The 
maximum temperature due to radio-activity could not, on an average, 
exceed 800° C. + mx. 
Since the pressure component is omitted, the fusion curves of 
minerals become straight lines parallel to the base, i.e. having for 
every depth the same ordinates. The point of intersection of, say, 
the labradorite line with any of the temperature curves gives the 
depth at which labradorite would fuse. Clearly, if none of the earth’s 
heat were due to radio-activity (curve IT), or if all its heat were due 
to radio-activity (curve III), none of the minerals represented in the 
diagram could fuse at all, whatever the depth. The temperature at 
which basalt fuses cannot be safely put below 1,200° C., and hence 
basalt magma, and for a similar reason peridotite magma, could not 
be produced under the conditions of curves II and III. Volcanic 
phenomena therefore bear witness to the verity of some such curve as 
I, and indeed curve I expresses almost the maximum temperatures 
that can occur in depth over wide areas and yet avoid the impossible 
extremes of universal vulcanism and no vulcanism. 
Curve I implies that basalt magma cannot usually be produced at ~ 
a depth of less than 140 km. But can basalt be supposed to exist at 
all at such adepth? The distribution of the radio-active elements 
indicates that it cannot. Indeed, even at a depth of 40 km.! basalt 
—or whatever may there be its equivalent—could not hold as much 
uranium and thorium as the basalts which have reached the surface, 
for if it did so the temperature gradient would certainly be higher 
than it is. ‘The great majority of the basaltic material of the earth 
seems, in fact, to be confined to an outer shell of about 30 km. in 
thickness. Below that depth, the peridotite minerals cannot hope to 
find a temperature at which they could melt within 200 or 300 km. 
of the surface. Beyond depths of this order peridotite may begin to 
approach its melting-point, and thus to attain the possibility of 
plastic flowage, but it is clear from curve I that actual fusion is not 
likely anywhere to be a widespread condition. If at any time fusion 
did become possible the fluid material would be squeezed out to 
higher levels, leaving a concentration of the unfused materials. It 
is therefore to be expected that the more fusible ferriferous olivines 
have long ago been expelled from the interior, leaving behind varieties 
richer in magnesium, accompanied perhaps by such refractory 
accessories as chromite. This view is strongly supported by the fact 
that ferriferous olivines are, on an’ average, richer in radium than 
those in which the forsterite molecule predominates. As between 
hypersthene and enstatite the same rule holds. 
Barrell’s views as to the physical condition of the asthenosphere 
are therefore seen to be completely in accord with the temperature 
1 70 km. would be the maximum depth possible if the continents were not 
capped with granites. 
