212 Arthur Holmes—Radio-activity. 
shell. Normally its temperature due to radio-activity is 264° C. 
(p. 111, Part II, March, 1915). The addition of a 10 km. blanket 
of sediment would increase this temperature, now at a depth of 
20km., to over 1,000°C. (10? : 20? = 264: 1056). The association 
of volcanic eruptions with sedimentation and the weakening of the 
crust preparatory to the upheaval of mountains along lines of heavy . 
sedimentation thus becomes easily explicable. 
The second case—that involving a deficiency of radio-activity— 
applies to areas which have undergone long denudation and have 
thus been deprived of part of their most richly radio-active rocks. 
If isostasy also comes into play, then regions originally at a consider- 
able depth and temperature may become exposed at the surface. 
Combined cooling and uplifting gives a double reason for the 
development of tensional stresses. Thus areas like Africa are rigefied 
and strengthened by cooling, and rent asunder by contraction and 
uplift. The existence of structural features such as rift valleys 
implies the development of tension clefts reaching down to considerable 
depths. ‘This in turn implies local relief of pressure and a corre- 
sponding reduction of the temperature at which the rocks below 
can attain fusion. Here, then, although the surface phenomenon is 
one of cooling and rigefaction, the effect in depth is to promote 
fusion, and vulcanism is again made possible. It thus appears that 
igneous activity may be controlled by two totally different classes of 
phenomena; in some regions by radio-active superheating, and in 
others by far-reaching relief of pressure. It is to be expected, then, 
that the mechanism of intrusion should be notably different in the 
two cases, and that at least partial differences in the composition of 
_ the resulting rocks should be observed between them. We have 
here a physical explanation for the undoubted core of truth contained 
in the conception of ‘ Atlantic’ and ‘ Pacific’ types of igneous rock— 
a conception that errs in detail rather than in principle; and one that 
has suffered by the rigidity of its expression rather than by its lack 
of verity. It is hoped in later contributions to this series of articles 
to discuss earth movements and their appropriate igneous associations 
in the light of the considerations briefly outlined above. 
8 
19. Isosrasy AND THE AGE oF THE HARTH. 
The title of this article provides the author with a suitable 
opportunity for discussing a recent paper by Dr. G. F. Becker.’ In 
it Dr. Becker correlates. recent developments in isostasy with others 
arising from the application of radio-activity to geological problems. 
He assumes that the undertow by which isostasy is maintained takes 
place at the basal level of isostatic compensation (1.e. at the depth of, 
say, 121km.), and that this level must also be the ‘‘ eutectic level ” 
at which fusion can most easily be accomplished. The ‘‘ eutectic 
level” 2, is calculated from the following equation connecting 2, 
and ¢, the age of the cooling earth :— 
ha rt (V— =O) ham ety [anPt (14) 
0-017 V 
1 ““ Tsostasy and Radio-activity’’: Bull. Geol. Soc. Am., yol. xxvi, 
pp. 171-204, 1915. 
