THE QUATERNARY PERIOD. 355 
deposits is therefore an extremely complicated one ; and as 
regards the age of even some of the most important of them, 
there still exists considerable difference of opinion. For our 
present purpose, it will be convenient to adopt a classifica- 
tion of the Post-Pliocene deposits founded on the relations 
which they bear in time to the great “ Ice-age” or “ Glacial 
period ;” though it is not pretended that our present know- 
ledge is sufficient to render such a classification more than a 
provisional one. 
In the early Tertiary period, as we have seen, the climate of 
the northern hemisphere, as shown by the Eocene animals and 
plants, was very much hotter than it is at present—partaking, 
indeed, of a sub-tropical character. In the Middle Tertiary or 
Miocene period, the temperature, though not so high, was still 
much warmer than that now enjoyed by the northern hemi- 
sphere ; and we know that the plants of temperate regions at 
this time flourished within the Arctic circle. In the later 
Tertiary or Pliocene period, again, there is evidence that the 
northern hemisphere underwent a further progressive diminu- 
tion of temperature ; though the climate of Europe generally 
seems at the close of the Tertiary period to have been if any- 
thing warmer, or at any rate not colder, than it is at the present 
day. With the commencement of the Quaternary period, 
however, this diminution of temperature became more de- 
cided ; and beginning with a temperate climate, we find the 
greater portion of the northern hemisphere to become gradu- 
ally subjected to all the rigours of intense Arctic cold. All 
the mountainous regions of Northern and Central Europe, of 
Britain, and of North America, became the nurseries of huge 
ice-streams, and large areas of the land appear to have been 
covered with a continuous ice-sheet. The Arcticconditions of 
this, the well-known “Glacial period,” relaxed more than once, 
and were more than once re-established with lesser intensity. 
Finally, a gradual but steadily progressive amelioration of tem- 
perature 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 de- 
posits collectively constitute the Post-Pliocene formations. 
Hence we may conveniently classify all the accumulations of 
