THK NEOZOIC AGES. 267 



dwarf birch of Scandinavia and the Highland hills, 

 and three willows, one of them the little Arctic and 

 Alpine creeping willow. Thus we have in the south 

 of England a transition in the course of the Pliocene 

 period, from a climate much milder than that of 

 Modern England to one almost Arctic in its character. 

 Our next topic for consideration is one of the most 

 vexed questions among geologists, the Glacial period 

 which immediately preceded the Advent of Man. In 

 treating of this it will be safest first to sketch the 

 actual appearances which present themselves, and 

 then to draw such pictures as we can of the conditions 

 which they represent. The most recent and super- 

 ficial covering of the earth's crust is usually composed 

 of rock material more or less ground up and wea- 

 thered. This may, with reference to its geological 

 character and origin, be considered as of three kinds. 

 It may be merely the rock weathered and decomposed 

 to a certain extent in situ ; or it may be alluvial 

 matter carried or deposited by existing streams or 

 tides, or by the rains ; or, lastly, it may be material 

 evidencing the operation of causes not now in action. 

 This last constitutes what has been called drift or 

 diluvial detritus, and is that with which we have now 

 to do. Such drift, then, is very widely distributed on 

 our continents in the higher latitudes. In the North- 

 ern Hemisphere it extends from the Arctic regions 

 to about 50 of north latitude in Europe, and as low 

 as 40 in North America; and it occurs south of 

 similar parallels in the Southern Hemisphere. Farther 



