operations be carried down to a great depth below the surface. The
greatest depth yet penetrated does not exceed three thousand feet, and
forms a very small advance towards the earth's centre, distant 4,000
miles.

Geologists, however, without penetrating far into the earth, have found
means for obtaining an insight for several miles into its interior
structure, and armed with hammer, chisel, and climbing hook, they
explore the beetling sea-cliff, traverse the deepest valleys, and scale
the highest mountains, carefully examining their formation, disposition,
and substance, and are thus enabled to obtain some knowledge of the
earth's stomach, as it were, by scrutinising the deposits and eruptive
ejectments on its surface. For example, we come to a mountain composed
of a particular substance with strata or beds of other rock lying
against its sloped sides; we, of course, infer that the substance of the
mountain dips away under the strata that we see lying against it.
Suppose that we walk away from the mountain across the turned-up edges
of the stratified rocks, and that for many miles we continue to pass
over other stratified rocks, all disposed in the same way, till we begin
to cross the opposite edges of the same beds; after which we pass over
these rocks all in reverse order, till we come to another extensive
mountain composed of similar materials to the first, and shelving away
under the strata in the same way; we should then infer that the
stratified rocks occupied a basin formed by the rocks of these two
mountains, and by calculating the thickness right through these strata
could say to what depths the rock of the mountain extended below. In
this way has the interior of the globe been examined, and its contents
and arrangement, for several miles below the surface, ascertained. The
result of such inspection we leave the author of the _Vestiges_ to
describe:--