|MadSci Network: Earth Sciences|
Sorry about the delay in answering this query. Jules Verne was writing at the end of the Victorian Era and many of his novels contained "state of the art" information for the time. There have been great advances in geological knowledge since then. Our knowledge of the structure of the Earth is based on the analysis of seismic records of earthquakes and underground explosions. This clearly demonstrates that the Earth has a core, a mantle and a crust. Considerations of the density and mass of the earth suggest that the core is made of a heavy metallic element. Iron is the metal that fits this idea best. The core is in two parts: an inner solid core and an outer liquid core. Movements within the liquid part create the magnetic field of the Earth. The mantle is fluid, but because of the immense pressures involved is only able to move slowly - think plasticene rather than water! Convection currents within the mantle cause the continents to move around. This is plate tectonics. In areas (mid ocean ridges and continental margins) hot mantle rises closer to the surface and the change in pressure causes the mantle material to become more fluid. This may reach the surface as lava at a volcano. The crust is the part of the Earth that the continents are made of. The crust is thin in relation to the mantle and core and floats on the mantle. Consult any Geology text book for more details. The book by Arthur Holmes (Principles of Physical Geology) is long in the tooth, but is still a very good starting point.
Try the links in the MadSci Library for more information on Earth Sciences.