REVIEWS. 733 



based ? There would seem to be no question as to the distinctness 

 of the Weybourn crag from the oldest glacial formations of the British 

 Isles. The only question would seem to be concerning the glacial 

 character of the Weybourn crag epoch. In favor of this view Professor 

 Geikie seems to make a strong case. Between the drift of the second 

 and third glacial epochs, the "lower" and "upper" bowlder clays of 

 England, there seems to be a well marked interglacial horizon. This 

 horizon represents an interval during which the land surface of Brit- 

 ain was considerably elevated. During the time of elevation the cli- 

 mate changed from arctic to temperate, and the land surface became 

 well clothed with vegetation, and was occupied by man and Pleisto- 

 cene mammalia. Following this condition of elevation, there was 

 submergence of the land to an undetermined extent. During the 

 submergence the climate changed from cold-temperate to arctic. The 

 increase in the severity of climate accompanying the submergence 

 resulted in a second mer de glace. Thus a very considerable period of 

 time, accompanied by a very considerable amelioration of climate, 

 and by very considerable changes of level, separated the lower drift 

 of Britain from the upper. 



The evidence on the basis of which the third epoch represented in 

 Scotland (the fourth of the full series) is separated from the preceding, 

 is drawn from several sources, (i) The mountain valley lake basins 

 are believed to represent the work of advancing ice, not the work of the 

 receding ice (valley glaciers) of the third epoch. (2) The bowlder clay 

 in the mountain valleys possesses a topography which is fresher (less 

 eroded) than that of the bowlder clay of the surrounding country. (3) 

 Above the valley moraines which are thought to mark the termini of 

 the glaciers of this epoch, there is an absence of till in the bottoms ot 

 the valleys, and on their lower slopes. Higher up the slopes bowlder 

 clay is present, as also in the same valleys below the moraines. The 

 absence of bowlder clay in the situations specified is attributed to the 

 erosive work of the mountain glaciers of the third (Scotland) epoch. 

 (4) The moutonnees, striae, etc., in the areas thought to have been cov- 

 ered by the local glaciers of the third epoch are fresher than the cor- 

 responding features elsewhere. (5) The coincidence of direction of ice 

 movement with the courses of the valleys in the third epoch is striking, 

 while during the preceding epoch the direction of ice movement was 

 independent of topography. (6) It is thought that after the last great 

 ice sheet (that of the third ice epoch) withdrew from Scotland, there 



