126 THE WONDERFUL CENTURY. 



Sea, and united with the glaciers of the Lake District, 

 Wales, and Ireland till almost continuous ice-sheets en- 

 veloped those countries also. Glacial striae are found up 

 to a height of 3500 feet in Scotland and 2500 feet in the 

 Lake District and in Ireland; while the Isle of Man was 

 completely overflowed, as shown by glacial stria3 on the 

 summit of its loftiest mountains. Erratics from Scan- 

 dinavia are found in great quantities on Flamborough 

 Head, mixed with others from the Lake District and 

 Galloway, showing that two ice-streams met here from 

 opposite directions. Erratics from Scotland are also 

 found in the Lake District, in North Wales, in the Isle 

 of Man, and in Ireland, from which the direction of the 

 moving ice can be determined. Great numbers of local 

 rocks have also been carried into places far from their 

 origin, and in every case this displacement is in the 

 direction of the flow of the ice as ascertained by the 

 other evidence never in the opposite direction. Each 

 great mountain area had, however, its own centre of local 

 dispersal, depending upon the position of greatest thick- 

 ness of the ice-sheet, which was not necessarily that of 

 the highest mountains, but was approximately the centre 

 of the main area of glaciation. Thus the centre of the 

 North Wales ice-sheet was not at Snowdon, but over the 

 Arenig mountains, which thus became a local centre of 

 dispersal of erratics. In Ireland, the mountains being 

 placed around the coasts, the great central plain became 

 filled with ice which, continually accumulating, formed 

 a huge dome of ice whose outward pressure caused mo- 

 tion in all directions till checked by the opposing motion 

 of the great Scandinavian ice-sheet. This strange fact 

 has been demonstrated by the work of the Irish Geo- 



