448 On the Thickness of the Crust of the Earth. 
Il. Archdeacon Pratt's demonstration that the crust of the earth 
cannot be very thin. 
Before discussing this question, I would premise that I am 
not an advocate for the idea, held by many physical geologists, 
that the earth has a crust, and that its crust is very thin. I 
believe it to be as unphilosophical to maintain it to be thin, as 
to hold it to be thick, and that no good reason can be given for 
either opinion. 
The following idea of the interior of the earth is one which I 
entertain myself with, but which I have no right to force upon 
another,viz. that the earth is composedof three layers—of granite, 
basalt or diorite, and meteoric iron and nickel, with an immense 
cavity in the centre caused by centrifugal foree—and that it is 
completely solid at present. ‘This, however, is a speculation, as 
unfounded as any of those I have attacked; and I must return 
to the demonstration that the earth’s crust cannot be very thin. 
This demonstration consists of three parts :— 
1. If very thin, the mountains would fall through. 
2. If very thin, the floor of the oceans would be forced up. 
_ 8. Ifvery thin, a semi-diurnal fracture would be caused by 
the tides of the fluid nucleus. 
I have already shown that Archdeacon Pratt’s mode of con- 
sidering the first of these questions is mechanically erroneous, as 
he supposes the mountain mass to be in a state of tension, 
whereas it is in a state of compression, and supports itself on the 
principle of the arch. 
In the second case, although the floor of the ocean is in a state 
of tension, if the voussoirs be supposed to coincide with the radii 
of the earth, yet we know so little of the real direction of the 
main joints, that it is unsafe to speculate about them, although 
their fan-shaped arrangement under the mountain axes would 
appear to indicate a provision to sustain the weight, or rather is 
itself a consequence of the supermcumbent weight. 
I prefer, however, to deny the validity of Archdeacon Pratt’s 
proof on the followmg grounds. As we know nothing of the 
interior of the earth, I am as well entitled as any other person 
to make hypotheses, and I accordingly make the followmg :— 
]. The mountain chains float, like icebergs, on the surface of 
the fluid nucleus, having deep roots penetrating far down into 
the denser fluid below. : 
2. The liquid displaced by the roots of the mountains, finds 
lodgement in cavities scooped out under the floors of the deep 
oceans, thus restoring the hydrostatical equilibrium of the crust, 
which is thinnest under the oceans, and thickest under the 
mountains. 
