circumstances
cannot be fully explained without assuming some things as proved, which
it will be the object of another part of this work to demonstrate, it
may be well to allude to them briefly in this place.

The first and greatest difficulty, then, consists in an habitual
unconsciousness that our position as observers is essentially
unfavorable, when we endeavor to estimate the nature and magnitude of
the changes now in progress. In consequence of our inattention to this
subject, we are liable to serious mistakes in contrasting the present
with former states of the globe. As dwellers on the land, we inhabit
about a fourth part of the surface; and that portion is almost
exclusively a theatre of decay, and not of reproduction. We know,
indeed, that new deposits are annually formed in seas and lakes, and
that every year some new igneous rocks are produced in the bowels of the
earth, but we cannot watch the progress of their formation; and as they
are only present to our minds by the aid of reflection, it requires an
effort both of the reason and the imagination to appreciate duly their
importance. It is, therefore, not surprising that we estimate very
imperfectly the result of operations thus invisible to us; and that,
when analogous results of former epochs are presented to our inspection,
we cannot immediately recognize the analogy. He who has observed the
quarrying of stone from a rock, and has seen it shipped for some distant
port, and then endeavors to conceive what kind of edifice will be raised
by the materials, is in the same predicament as a geologist, who, while
he is confined to the land, sees the decomposition of rocks, and the
transportation of matter by rivers to the sea, and then endeavors to
picture to himself the new strata which Nature is building beneath the
waters.

_Prejudices arising from our not seeing subterranean changes._--Nor is
his position less unfavorable when, beholding a volcanic eruption, he
tries to conceive what changes the column of lava has produced, in its
passage upwards, on the intersected strata; or what form the melted
matter may assume at great depths on cooling; or what may be the extent
of the subterranean rivers and reservoirs of liquid matter far beneath
the surface. It should, therefore, be remembered, that the task imposed
on those who study the earth's history requires no ordinary share of
discretion; for we are precluded from collating the corresponding parts
of the sy