Page:Popular Science Monthly Volume 35.djvu/426

406, found in morainic deposits and associated kames 1,100 to 1,350 feet above the sea, on Three Rock Mountain, near Dublin, on Moel Tryfan in northern Wales, and near Macclesfield in Cheshire, which have been generally considered by British geologists as proof of marine submergence to the depth of at least 1,350 feet. These shells and fragments of shells, as Lewis has shown, were transported to their present position by the currents of the confluent ice-sheet, which flowed southward from Scotland and northern Ireland, passing over the bottom of the Irish Sea, there plowing up its marine deposits and shells, and carrying them upward as glacial drift to these elevations, so that they afford no testimony of the former subsidence of the land. The ample descriptions of the shelly drift of these and other localities of high level, and of the lowlands of Cheshire and Lancashire, recorded by English geologists, agree perfectly with the explanation given by Lewis, which indeed had been before suggested, so long ago as in 1874, by Belt and Goodchild. This removes one of the most perplexing questions which geologists have encountered, for nowhere else in the British Isles is there proof of any such submergence during or since the Glacial period, the maximum known being 510 feet, near Airdrie, in Lanarkshire, Scotland. At the same time the submergence on the southern coast of England was only from ten to sixty feet, while no traces of raised beaches or of Pleistocene marine formations above the present sea-level are found in the Orkney and Shetland Islands. The work and writings of Prof. Lewis emphasize the principle that glacially transported marine shells and fragments of shells, which occur in both the till or bowlder-clay and the modified drift in various parts of Great Britain, are not to be confounded with shells imbedded where they were living, or in raised beaches, for only these prove the former presence of the sea.

"The drift deposits of England south of the terminal moraines traced by Lewis were regarded by him as due to floating ice upon a great fresh-water lake, held on the north by the barrier of the ice-sheet which covered Scotland, northern England, and the area of the North Sea, and on the southeast by a land-barrier where the Strait of Dover has since been eroded. Under this view he attributed the formation of the Chalky bowlder-clay in East Anglia and of the purple and Hessle bowlder-clays in Lincolnshire and much of Yorkshire to lacustrine deposition, and believed that there was only one advance and recession of the ice-sheet. But shortly after the British Association meeting in 1887 his observations on Frankley Hill in Worcestershire and thence westward led him to accept the conclusion, so thoroughly worked out by other glacialists both in America and Great Britain, that there were two principal epochs of glaciation, divided by an