The Intericr of the Earth — Claypole. 2^ 
Mr. Davison calls the "layer of no strain." He shows that it 
sinks deeper with time. At the present day he places it at the 
depth of about five miles. It forms a limit between the bend- 
ing and crushed layers above it and the squeezed and flattened 
layers below it. 
If we understand Mr. Davison aright his conclusions are — 
first that above this level of "no strain" all the couches up to the 
surface are in a state of compression in consequence of their 
constant sinking, without equivalent contraction, to lower and 
lower levels, or to the surfaces of spheres of less and less radius. 
And secondly that below this plane of no strain the couches are 
in a state of extension — that is are being squeezed out because 
they are descending into a smaller space while their contraction 
exceeds this narrowing and would leave chinks or gaps were it 
not for the immense weight above them under which they are 
plastic and which squeezes them out laterally so as to fill these 
chinks or rather to prevent their formation.' The total vertical 
descent thus obtained of the upper surface of these extended 
couches exactly equals that through which the layer of "no 
strain" is ready to descend without extension or compression by 
its own contraction from loss of heat; while its horizontal 
linear contraction exactly equals the difference between its own 
circumference and that of the shell into whose place it is on the 
point of descending. Its continuity is therefore preserved. 
It is obvious that the result obtained must depend on twa 
primary data — the temperature of the surface at the time of 
consolidation and the time that has since elapsed. The higher 
the temperature of consolidation the longer must the cooling 
have continued to obtain the present temperature of the crust, 
while the same result would be reached by assuming a lower 
original temperature and combining it with a shorter period of 
cooling. 
Mr. Davison assumes, following, he says. Sir W. Thompson, 
about 175,000,000 years as the time that has elapsed since the 
consolidation of the surface. On this datum he tells us that the 
cooling by radiation ceases, that is becomes infinitesimally small 
1 It is possible that in the highest portion of this sphere such chinks or 
crevices may be actually formed, but it is scarcely likely when we con- 
sider the enormous pressure, which is capable of crushing any known 
rock. 
