Arthur Holmes—Radio-activity. 269 
what is observed at the surface. If, therefore, the pressure 
component could be left out of the discussion altogether, and 
a disturbing factor thus avoided, a considerable advantage would be 
gained. Now, for the present purpose, this can readily be done, for 
it is required to compare the fusion-points of certain minerals and 
rocks! with the temperatures at depths appropriate to their occurrence 
within the earth. Let us suppose that for a given mineral or rock 
the rise of the fusion-point at a depth 2 is mx. Then the pressure 
component of temperature at that depth willalso be mz. The fusion- 
point may be expressed as S + mx where S is the fusion-point at the 
surface, and the temperature as 7’+ mx where 7'is the temperature 
due to other factors than the pressure component. Consequently, in 
plotting fusion-point and temperature curves the differential relation 
of one to the other will not be affected by plotting S and 7’respectively 
instead of S + mx and 7+ mz. Thus, at every depth, «, the 
appropriate but at present unknown value of mz, may be omitted 
without detriment, and with the advantage that a doubtful extra- 
polation is completely avoided. 
1800;C 
Pa Sy SEES NWS EE PG EE I Si barn ip MR nL MCS ON CM Ree eG 
LE 
{600 
: Enstatite 
Olivine 
Diopside 
1200} abradorite 
r 
800° 
F 
~ x 
" ‘\ 
400° <i 
: 5 
XN 
x 
i » I Depth in Kilometres . 
°o 100 200 Zi 400 500 600 700 
Fic. 1.—The distribution of temperature in depth in relation to the fusion- 
points of minerals. _ 
I. Temperature gradient due to radio-activity and the earth’s initial thermal 
condition in the respective ratio of 3: 1. 
Ii. Temperature gradient due to the earth’s initial thermal condition alone. 
Ill. Temperature gradient due to radio-activity alone. 
In each ease the pressure component mx is omitted. 
In the adjoining diagram the temperature 7’ is plotted in degrees 
Centigrade against depth in kilometres. Curve I represents the 
distribution of temperature based on the discussion in Part II 
(omitting the pressure component mx); thatis to say, the temperature 
gradient is considered on an average to be due to radio-activity and 
secular cooling in the ratio of approximately 3 to 1. Curve II 
1 By the expression ‘‘fusion-point of a rock’’ is meant the minimum 
temperature at which the whole of the rock passes into the non-crystalline 
condition. 
