470 Reviews — H. Carvill Leivis — Glacial Geology. 



historical account of her husband's work. Professor Lewis believed 

 that the division of the drift into Lower Boulder-clay, Middle 

 Glacial sands, and Upper Boulder-clay, has yet to be established ; 

 and thought it probable that the Upper and Lower Boulder-clays in 

 Lancashire may be the same deposit. Science is always in danger 

 from hasty generalization of local phenomena ; and those who are 

 familiar with the Eastern Counties sections of the Upper Boulder- 

 clay have probably always held that the three-fold division of the 

 drift is essentially local, and that there has never been any demon- 

 stration of its universality. 



Professor Lewis called attention to the fresh- water origin of many 

 deposits regarded as glacial, pointing out that when the courses of 

 rivers were dammed by advancing or retreating ice, morainic lakes 

 would be formed, in which icebergs would float ; and as a necessary 

 corollary he concluded that areas free from glaciation existed in 

 between the glaciers. He endeavoured to define these glaciated 

 areas by tracing their terminal moraines, and maintained that every 

 glacier could be defined by this evidence in the time of its maximum 

 development. A glacier may form a terminal moraine, but if the 

 history of the glacier be a story first of augmentation and then of 

 diminution, the retreat of the glacial moraine may give rise to 

 appearances which are not quite in harmony with the theoretical 

 position of the parent ice. There can be no doubt that enormous 

 masses of Boulder-clay have been denuded since the Glacial period, 

 and the residue of the boulders is only to be found in post-Glacial 

 deposits. And one of the problems of the future, as of the past, 

 is to discover the relation of the snow waters upon the surface of 

 the land to the erosion of the Boulder-clay. Por it is always 

 towards the river basins that the clayey matter disappears, and the 

 Boulder-clay passes into a hill gravel which shows some evidence 

 of water-action. For a long time a submergence in the middle of 

 the Glacial period had been looked upon as practically demonstrated, 

 but of this submergence Professor Lewis found at first no evidence, 

 or only such evidence as would imply a submergence of 100 feet on 

 the east coast, and 150 feet on the west coast of England. 



The shell beds in the Boulder-clay which occur at great elevations 

 were regarded as having been pushed up bodily out of the Irish Sea 

 into the positions in which they are found. 



The fossiliferous deposits at a height of 400 to 450 feet, he 

 supposed to be transported by glaciers which formed an ice-dam, 

 helping to enclose an extra morainic lake ; and in the same way 

 the beds at Moel Tryfaen are regarded as examples of glacial 

 transport. There is some appearance of Professor Lewis having 

 arrived at the conclusion that there was a second and older Glacial 

 epoch on the evidence furnished by the sections at Frankley Hill 

 and California, near Birmingham, between the 500 feet and 900 

 feet levels, where he found great abundance of Welsh felsites and 

 slates. Since the first Glacial epoch he believed great erosion to 

 have occurred before the second Glacial period. He remarks that 

 the Lower Till at California, if not made by a glacier, was made by 



