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Page:Popular Science Monthly Volume 44.djvu/806

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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 southeastward to near Whalley. Along the same line are found bowlders 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 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 part of Anglesea, showing how the ice-flows 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.[1]

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

  1. Nature, vol. xlvii, p. 464.