CHAP. xii. GEOLOGICAL PRINCIPLES. 125 



lower slopes of the Alpine chain toward Bern on one 

 side and Geneva 011 the other, while the French geolo- 

 gists have traced them down the Rhone valley seventy 

 miles from Geneva, and also more than twenty miles 

 west of the Jura, thus proving that at the lowest portion 

 of that chain the glacier flowed completely over it. In 

 all these cases the blocks can be traced to a source cor- 

 responding to their position on the theory of glacier 

 action. Some of these rocks have been carried consider- 

 ably more than 200 miles, proving that the old glacier 

 of the Rhone extended to this enormous distance from its 

 source. 



In our own islands and in North America these va- 

 rious classes of evidence have been carefully studied, the 

 direction of the glacial striae everywhere ascertained, 

 and all the more remarkable erratic blocks traced to 

 their sources, with the result that the extent and thick- 

 ness of the various glaciers and ice-sheets are well de- 

 termined and the direction of motion of the ice ascer- 

 tained. The conclusions arrived at are very extraor- 

 dinary, and must be briefly indicated. 



In Great Britain, during the earlier and later phases 

 of the ice age, all the mountains of Scotland, the Lake 

 District, and Wales produced their own glaciers, which 

 flowed down to the sea. But at the time of the culmi- 

 nation of the Glacial Epoch the Scandinavian ice-sheet 

 extended on the southeast till it filled up the Baltic Sea 

 and spread over the plains of northwestern Europe, and 

 also filled up the North Sea, joining the glaciers of Scot- 

 land, forming with them a continuous ice-sheet from 

 which the highest mountains alone protruded. At the 

 same time this Scotch ice-sheet extended into the Irish 



