4 o6 THE POPULAR SCIENCE MONTHLY. 



ated, found in morainic deposits and associated kanies 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 Ches- 

 hire, 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 con- 

 fluent ice-sheet, which flowed southward from Scotland and north- 

 ern Ireland, passing over the bottom of the Irish Sea, there plow- 

 ing up its marine deposits and shells, and carrying them upward 

 as glacial drift to these elevations, so that they afford no testi- 

 mony of the former subsidence of the land. The ample descrip- 

 tions 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 

 1871, by Belt and Goodchild. This removes one of the most per- 

 plexing 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 Pleisto- 

 cene 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 Brit- 

 ain, 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 An- 

 glia 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 observa- 

 tions 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 intergla- 



