THE MEDITERRANEAN NATURALIST 
47 
ciently potent to account for such grand features 
as mountain ranges, though Captain Hutton, of 
Christchurch, X ew Zealand, pushed the reasoning 
further in a very suggestive paper published some 
years back in the Geological Magazine. 
The contraction theory, as stated by its ex- 
ponents, was doubtless sufficient — and, indeed, as 
Lyell pointed out, much more than sufficient — to 
account for the upheaval of all the known ranges 
considered as so many cubic miles of rock. 
Notwithstanding that so many mathematicians 
and physicists had worked at the problem, the 
underlying idea was that whole of the solid crust 
of the earth — a very vague term — must be in com- 
pression by the cooling of the nucleus, the favourite/ 
illustration being taken from the apple in which 
the core consisting of the fruit contracts by drying, 
while the rind remains of the same dimensions, 
or nearly so and is consequently thrown into 
wrinkles. Who does not remember the familiar 
“wrinkled hand’’ photographed in Nasmyth and 
Carpenter’s work on the Moon, as another illus- 
tration, though the appositeness of the analogy is, 
to say the least, doubtful. That most supporters 
of the theory without further inquiry accepted the 
position that all the rigid crust of the earth must 
be in compression is sufficiently evidenced by the 
remarks in Prestwich’s Geology , vol. II., p. 540 
(1887), where even the possibility of a shell 800 
miles thick being in compression is discussed but 
rejected. Prestwich, along with some other 
geologists, thinks that the hard crust of the earth 
rests upon a semi-fluid zone, and that the whole 
of the crust, whatever thickness it may be, is in 
compression. Although this is not always ex- 
pressly stated by the upholders of the contraction 
theory, it, so far as ray acquaintance with the 
subject extends, inferentially forms the basis of 
their reasoning. At any rate, whether considered 
partially fluid or solid throughout, they assume 
the earth to be divided into a contracting nucleus 
and an uncontracting crust, and we are left to put 
any quantitative value we like upon either. This 
may be seen by a reference to Geikie’s Pert- Hook 
of Geology , and Green’s Physical Geology, both 
justly considered standard geological works. 
In my Origin of Mountain Range*, published 
towards the end of 1886, I pointed out that the 
crust of the earth, whatever thickness we assign to I 
it, must, on the assumption of secular cooling, 
excepting at the surface, be itself contracting. If 
we assume the earth to be solid throughout, and 
to be divided into a series of thin shells, each 
under shell, commencing with the one immediately 
below the surface, will circumferentially contract 
more than the one above or enclosing it until the 
zone of maximum contraction is reached, when the 
underlying shells will contract less than those 
overlying them, till a depth is attained at which 
there is practically no contraction. As the earth is 
cooling from the outside, and the mean increase of 
heat downwards is about 1° in 60 feet — and even 
less, as estimated by some authorities — a simple 
calculation will show that the cooling of the body 
of the. earth is now of only a superficial character, 
not, practically speaking, penetrating above 200 
miles. From this it will be seen that the radial 
contraction of the earth is limited to the depth to 
which it has cooled from the exterior, and there- 
fore the apple illustration, where the core contracts 
throughout , is quite misleading. 
Vague general statements instead of quantitative 
determinations are responsible for much incon- 
clusive geological reasoning. 
It may be almost taken as an axiom that 
physical without quantitative reasoning is, more 
often than not, quite misleading. But, to return 
from this digression, it is readily seen that the 
radial contraction of the earth must be of limited 
quantity, and, this being so at a certain calculable 
depth, the mean rate of circumferential contrac- 
tion must be equal to the mean rate of radial 
contraction. 
The shell at this depth, whether the nucleus be 
fluid or solid, will be neither in compression nor 
tension, while all above will be in compression and 
all below' in tension. Speaking exactly, this neutral 
zone may be looked upon as a shell infinitesimally 
thin, and it has since been appropriately named by 
Rev. 0. Fisher the “level of no strain.” The whole 
of this reasoning with an illustrative diagram, may 
be seen in Chapter XL of my Origin of Mountain 
Ranges, and it is there shown that this zone or 
level-of-no-strain in our globe, cannot be situated 
many miles below' the surface. 
Since my chapter was written, the subject has 
been investigated by Davison, Darwin, and Fisher. 
The first of these mathematicians contributed a 
