Arthur Holmes—Radvo-activity. OT 
distribution expressed in curve 1. The weakest part of the outer 
regions of the earth must be located below the zone of compensation, 
and this conclusion may be used as an additional argument in favour 
of the existence of an asthenosphere, or, accepting the asthenosphere, 
it may be used as a rough corroboration of the distribution of 
temperature here advocated. 
18. VuLCANISM IN RELATION TO MouNTAIN-BUILDING AND RIGEFACTION. 
It may be thought that the preceding discussion leads on to the 
deduction that, whatever vulcanism may have gone on in the past, 
none ought to be possible at the present day. This is not so, for 
until now an average distribution of the radio-active elements has 
alone been considered. In relation to worldwide phenomena such as 
the maintenance of isostasy between continents and oceans, the use 
of average conditions is justified by the broadness of the problems 
involved. In relation to more localized phenomena such as mountain- 
building, block faulting, and vulcanism, departures from average 
conditions become critically important. The departures may, of 
- course, be of two kinds, those in which the distribution of the radio- 
active elements and the dependent temperatures in depth are locally 
in excess of the average, and those in which the radio-active elements 
and deep-seated temperatures are deficient. 
Let us consider the first of these cases. In regions of heavy 
sedimentation, a thick blanket of rocks considerably richer in radium 
and thorium than basalt (Part I, p. 63, February, 1915) is super- 
imposed on the previously existing ‘‘radio-active layer”. As 
_ Professor Joly has shown, when ‘‘ we take into account that in the 
radium content of the sediments there is a source of heat competent 
to bring the geotherms closer to the surface in the region of deposition 
than elsewhere in the surrounding crust ”’,' the possibility of weakness 
and even of fusion at moderate depths becomes clear. ‘The rise of 
temperature at and below the base of the accumulating sediments 
is much greater than one would think possible, for the basal 
temperature due to radio-activity is proportional to the square of the 
thickness of the uniform radio-active layer above it. ‘To illustrate 
the effect by a concrete example, let us take the figures referring to 
average igneous rock given on pp. 66 and 67 of Part I (February, 
1915). Ifthe distribution of radio-activity were uniform the depth 
of the radio-active layer would be 30 km., and the basal temperature 
570° C. The addition of a thickness of 10 km. of sedimentary rock 
would then increase the thickness of the radio-active layer to 40 km. 
and the new basal temperature would become 1,005° C. (30? : 40?= 
570: 1005). Since the distribution of radio-activity is not uniform, 
but decreases in depth, and therefore continues to much greater 
depths than 30 km., so the rise of temperature due to sedimentation 
is considerably greater than that given in the calculation just made. 
An exact calculation of the effect on an exponential distribution 
would be difficult, if not impossible, but the magnitude of the rise in 
temperature may be appreciated by considering only the outer 10 km. 
1 Radio-activity and Geology, 1909, p. 94; see also The Birth-time of the 
World, 1915, p. 116 et seq. 
