The Interior of the Earth — Claypole. 31 
We will defer consideration of one or two of Mr. Davison's 
data for the present, in order to regard from a geological stand- 
jDoint the already quoted statements of the mathematician. 
While no doubt can exist concerning the reality of this layer 
of "no strain" within the crust of the earth, yet a careful con- 
sideration of the whole subject and of the inferences deducible 
therefrom, suggests to the physical geologist some reasons for 
thinking that in determining its position the mathematician has 
placed it too near the surface. 
We take Mr. Davison's meaning to be that it lies at a depth 
of five miles beneath the solid crust of the sphere, disregarding 
the waters of the ocean, and that it therefore roughly follows 
the inequalities of the surface. This is indeed obvious, for the 
ice-cold water 4:hat fills the abysses of the ocean must by con- 
duction and convection chill the crust below it at least as much 
as the continental areas are cooled by radiation. This seems to 
be confirmed by the fact that the beds of the great oceans are 
denser than the continents. We may therefore conclude that 
the layer of "no strain" not only rudely follows but somewhat 
exaggerates the contour of the surface, and lies at a less depth 
beneath the dry land than beneath the deep sea. If then it lie 
at the average depth of five miles it must under the continents 
lie at somewhat less than that depth. 
Further, from the very nature of the case it was in the past 
during the palaeozoic ages for example, nearer the surface than 
it is now. Yet again from its own nature — being a shell of 
perfect freedom from all stress and strain and consequently from 
all motion except the gentle subsidence caused by the contrac- 
tion of the shells below it — it can never be elevated into anti- 
clines or depressed into synclines. 
Notwithstanding this inference we have in Pennsylvania 
strata now exposed, which at the close of the palaeozoic era were 
buried, not only five, but even eight miles deep. The Trenton 
limestone, for example, was covered in some places with from 
25,000 to 35,000 feet of newer strata. Yet it was subjected to 
compression and contortion and has been thrust up from that im- 
mense depth into huge arches and troughs. And there is little 
reason to doubt that beds lower still were included in those 
movements that produced the Appalachian revolution and closed 
the palaeozoic era in North America. 
