788 THE POPULAR SCIENCE MONTHLY. 



tioned as occurring at Flamborough Head, we find that it has been 

 carried northward as far as the Solway Frith, and eastward to the 

 Eden Valley in great quantity and over a wide area. Thence 

 can be traced a line of bowlders of this rock over the high plateau 

 of Stainmoor into the valley of the Tees, and onward round the 

 coast by Scarborough to Holderness, while a branch descends 

 southward along the valley of the Ouse to York. Coming back 

 to its source on Shap Fell, a train of bowlders of the 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 bowl- 

 ders 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 Cum- 

 berland and North Wales. This is indicated by the fact that 

 south of this line are scattered immense quantities of erratics, 

 both from the southwest 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 jDart of Anglesea, showing how the iceflows divided on either 

 side of the mountain mass of North Wales. 



The center 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 southeastward to the country west 

 of Birmingham. 



In the Isle of Man are found many erratics from Galloway 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 bowlder 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, County Dublin, on the seashore.* 



The case of the bowlders in the Isle of Man, which have been 

 carried nearly eight hundred feet above their source, has already 

 been mentioned, but there are many other examples of this phe- 

 nomenon in our islands ; and as they are of great iinportance 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 to 

 be found in situ; and he also noticed several deposits of limestone 



* Nature, vol. xlvii, p. 4(54. 



