countries, has enabled me to mark out not only
a considerable area, previously little known, in which tertiary
formations occur; but also a still wider expanse, over which the
northern drift, and erratic blocks with occasional marine shells, are
traceable. The southern limits of these glacial deposits in Russia and
Germany indicate the boundary, so far as we can now determine it, of the
northern ocean, at a period immediately antecedent to that of the human
race.

I was anxious, even in the title of this map, to guard the reader
against the supposition that it was intended to represent the state of
the physical geography of part of Europe at any _one point of time_. The
difficulty, or rather the impossibility, of restoring the geography of
the globe as it may have existed at any former period, especially a
remote one, consists in this, that we can only point out where part of
the sea has been turned into land, and are almost always unable to
determine what land may have become sea. All maps, therefore, pretending
to represent the geography of remote geological epochs must be ideal.
The map under consideration is not a restoration of a former state of
things, at any particular moment of time, but a synoptical view of a
certain amount of one kind of change (the conversion of sea into land)
known to have been brought about within a given period.

It may be proper to remark that the vertical movements to which the land
is subject in certain regions, occasion alternately the subsidence and
the uprising of the surface; and that, by such oscillations at
successive periods, a great area may have been entirely covered with
marine deposits, although the whole may never have been beneath the
waters at one time; nay, even though the relative proportion of land and
sea may have continued unaltered throughout the whole period. I believe,
however, that since the commencement of the tertiary period, the dry
land in the northern hemisphere has been continually on the increase,
both because it is now greatly in excess beyond the average proportion
which land generally bears to water on the globe, and because a
comparison of the secondary and tertiary strata affords indications, as
I have already shown, of a passage from the condition of an ocean
interspersed with islands to that of a large continent.

But supposing it were possible to represent all the vicissitudes in the
distribution of land and sea that have occurred during the terti