Finally, a gradual but steadily progressive
amelioration of temperature took place; the ice slowly gave way,
and ultimately disappeared altogether; and the climate once more
became temperate, except in high northern latitudes.
The changes of temperature sketched out above took place slowly
and gradually, and occupied the whole of the Post-Pliocene period.
In each of the three periods marked out by these changes--in
the early temperate, the central cold, and the later temperate
period--certain deposits were laid down over the surface of the
northern hemisphere; and these deposits collectively constitute the
Post-Pliocene formations. Hence we may conveniently classify all
the accumulations of this age under the heads of (1) _Pre-Glacial_
deposits, (2) _Glacial_ deposits, and (3) _Post-Glacial_ deposits,
according as they were formed before, during, or after the "Glacial
period." It cannot by any means be asserted that we can definitely
fix the precise relations in time of all the Post-Pliocene deposits
to the Glacial period. On the contrary, there are some which
hold a very disputed position as regards this point; and there
are others which do not admit of definite allocation in this
manner at all, in consequence of their occurrence in regions
where no "Glacial Period" is known to have been established.
For our present purpose, however, dealing as we shall have to do
principally with the northern hemisphere, the above classification,
with all its defects, has greater advantages than any other that
has been yet proposed.
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