268 Arthur Holmes—Radio-activity. 
involves a paring away of the upper surface of the lands, and an 
accumulation of the resulting debris around their borders. The land 
columns become lighter; the sea floor is weighed down. An equally 
certain fact, attested by many an eloquent chapter of geological 
history, is that during the transfer of load from continent to ocean 
floor the former rises and the latter sinks. What is the hidden 
mechanism that restores the isostatic balance? Corresponding to the 
lateral movement of sediment there must be a lateral counter- 
movement in the rocks far below. At first Hayford considered that 
this undertow took place within the zone of compensation, but Barrell’ 
has convincingly shown that this view is untenable, and Hayford? in 
his latest pronouncement has apparently abandonedit. ‘he undertow 
must be below the zone of compensation, and consequently the matter 
at greater depths must be somewhat plastic. Moreover, to maintain 
an isostatic balance between segments of the lithosphere so extensive 
as continents and ocean basins the plastic zone—the asthenosphere— 
must be very thick, certainly several hundreds of kilometres. 
Quite independent evidence comes from another branch of geophysics. 
Tt has sometimes been assumed that a viscous magmatic zone might 
exist within the earth between a rigid crust and a rigid core. 
Schweydar * has tested this theory by analysing the results of earth- 
tide measurements made with the horizontal pendulum. He proved 
conclusively that a magmatic zone of the kind imagined could not 
exist, and showed that the results pointed—as one alternative—to the 
existence of a slightly plastic zone extending downwards from a depth 
of 120 km. and fading out some 600 km. below. 
The existence of the asthenosphere is thus supported by arguments 
based on three different lines of investigation. In the remaining 
part of this paper its existence will be accepted, and used asa criterion 
by which to test the distribution of temperature within the earth that 
was deduced in Part II. 
17. Tue Temperature Curve TESTED BY Isosrasy. 
The temperature at any depth below the earth’s surface has been 
regarded as the sum of three components—(1) that due to the initial 
temperature at or near the surface, and since modified by cooling ; 
(2) that implied by the variation of the fusion-point with depth, as 
controlled by pressure; and (3) that due to the local distribution 
of the radio-active elements. — | 
These three components are mathematically expressed for an 
depth 2 in equations 9 and 18 on p. 111 of Part II (March, 1919). 
In calculating the list of temperatures there tabulated, it was. 
assumed that the downward increase of temperature due to pressure 
was 0:00005° C. per cm., or 5° C. per km. Such an assumption 
cannot, of course, be applied satisfactorily to any considerable depth. 
We do not know to what extent the law connecting pressure with 
a rise of the fusion-point of a mineral is linear, and moreover the 
distribution of minerals in depth cannot be a simple continuation of 
1 Barrell, op. cit., p. 677, 1914. 
2 Hayford, Proc. Am. Phil. Soc., vol. liv, p. 298, 1915. 
3 Verdff. Preuss. geodat. Inst., No. 54, 1912. 
