348 HISTORICAL PALEONTOLOGY. 



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 

 this age under the heads of (i) Pre-Glacial deposits, (2) Glacial 

 deposits, and (3) Post-Glacial deposits, according as they were 

 formed before, during, or after the " Glacial period." It can- 

 not 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 classifi- 

 cation, with all its defects, has greater advantages than any 

 other that has been yet proposed. 



I. PRE-GLACIAL DEPOSITS. The chief pre-glacial deposit of 

 Britain is found on the Norwalk coast, reposing upon the Newer 

 Pliocene (Norwich Crag), and consists of an ancient land-sur- 

 face which is known as the " Cromer Forest-bed." 



This consists of an ancient soil, having embedded in it the 

 stumps of many trees, still in an erect position, with remains 

 of living plants, and the bones of recent extinct quadru- 

 peds. It is overlaid by fresh-water and marine beds, all the 

 shells of which belong to existing species, and it is finally sur- 



