The interior of the earth is completely beyond the reach of direct observation and what is known, or may be reasonably inferred, as to its physical constitution, is derived from various lines of indirect evidence. The deepest boring ever made is less than 1/3000 part of the earth's radius, and we have no experience with such enormous pressures as obtain within the mass of the globe, and can therefore form but imperfect conceptions of their effects.

From observations with the pendulum and plumb-line it is calculated that the specific gravity of the earth as a whole is 5.6, while the average specific gravity of the rocks which form the accessible parts of the crust is only 2.6. It follows that the interior of the globe is composed of much denser materials than the superficial portion, and this fact, together with the phenomena of terrestrial magnetism, has led many to the belief that the earth is substantially a globe of iron. This inference is also supported by the occurrence of native, or unoxidized, iron in certain igneous rocks.