THE QUATERNARY PERIOD. 335 



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 Arctic conditions 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 



