90 STUDIES, SCIENTIFIC AND SOCIAL chap. 



same rock has been traced southward in a curving line, 

 passing the east side of Morecambe Bay near Lancaster, 

 and thence sparingly south-eastward to near Whalley. 

 Along the same line are found boulders of peculiar granites 

 from Eskdale and Buttermere, marking the line of junction 

 of the northern ice-sheet with that which filled up the 

 Irish Sea and pressed inward between the glaciers of 

 Cumberland and North Wales. This is indicated by the 

 fact that south of this line are scattered immense quanti- 

 ties of erratics, both from the south-west of Scotland and 

 the Lake District, spreading over the whole of the low 

 country as far as Bridgnorth and Wolverhampton, and 

 eastward to the Derbyshire highlands. These same 

 erratics are found round the north coasts of Wales and 

 part of Anglesea, showing how the ice-flows divided on 

 either side of the mountain mass of North Wales. 



The centre of the great glacier sheet of North Wales 

 appears to have been over the Arenig Mountains, whence 

 erratics of a peculiar volcanic rock have been traced to the 

 north and east, mingling with the last-described group : 

 while a distinct train of these Welsh erratics stretches 

 south-eastward to the country west of Birmingham. 



In the Isle of Man are found many erratics from Gallo- 

 way, and a few from the Lake District. But the most 

 remarkable are those of a very peculiar rock found only on 

 Ailsa Craig, a small island in the Frith of Clyde, and a 

 single boulder of a peculiar pitchstone found only in the 

 Isle of Arran. The Ailsa Craig rock has also been found 

 at Moel Tryfaen on the west side of Snowdon, and more 

 recently at Killiney, co. Dublin, on the seashore.^ 



The case of the boulders in the Isle of Man, which have 

 been carried nearly 800 feet above their source, has already 

 been mentioned ; but there are many other examples of 

 this phenomenon in our islands, and as they are of great 

 importance in regard to the general theory of glacial 

 motion a few of them may be noted here. So early as 

 1818 Mr. Weaver described a granite block on the top of 

 Cronebane, a slate hill in Ireland, and several hundred 

 feet higher than any place where similar granite was 



^ Nature, vol. xlvii., p. 464, 



