456 MR T. J. JEHU ON 



crystalline rocks are sometimes decayed to a depth of more than 300 feet ; and in parts 

 of North America "the depth of disintegration appears gradually to increase southward 

 from the limits where the country has been glaciated by ice-sheets during the Glacial 

 Epoch." A similar superficial decay has been observed in our own country in the 

 granite and phyllite of Cornwall and Devon, which lay outside the limits of glacial 

 action. In many places they show a deep cover of rotted rock, and so " afford some 

 indication of what may have been elsewhere the condition of Britain before the period 

 of glaciation." This rotting of the solid rock through the prolonged operations of the 

 weathering processes is a fact of some importance. The rocks would waste irregularly 

 according to their varying powers of resistance. Some lakes may lie in hollows scooped 

 out of the decomposed material by glaciers ; or a hollow thus started by sub-aerial waste 

 may subsequently get enlarged and deepened by the moving ice. The relative hardness 

 of the rocks, on the one hand, and the thickness of the ice, on the other, would have an 

 important effect on the work done. 



Mr J. E. Marr, in his work on the Scientific Study of Scenery, has pointed out 

 that in a region where the climate is humid, small rock-basins may be formed by the 

 unequal weathering of rocks which are covered by vegetation. He has observed in 

 the English Lake-District little basins of this character, a few feet or yards in diameter, 

 and in every stage of formation. 



But during recent years important facts have been brought to light which go to 

 show that glaciers are powerful agents of erosion, and are actually able to quarry the 

 beds over which they flow. Much of this new evidence is summarised by Professor 

 James Geikie in his recent book on Earth- Sculpture. The lines of evidence are both 

 indirect and direct. The indirect evidence is obtained by a study of ground-moraines. 

 Professor Geikie shows that the existence of ground-moraines does not depend on the 

 presence of superficial moraines. It may be true that in some of the Swiss glaciers 

 much of the infra-glacial detritus comes chiefly from superficial sources, but this cannot 

 be the case in Norway and Greenland. The Norwegian glaciers, as compared with those 

 of the Alps, are almost devoid of rock-debris on their surface. Nevertheless they extrude 

 ground-moraines. This is even more clearly the case in Greenland, where the ice-sheet, 

 though free from superficial moraines, shows ground-moraines where it terminates on land. 

 ^'Nansen, for example, tells us that at Austmannatjern, where he left the inland ice 

 after his famous traverse, enormous accumulations of moraines were seen. These were 

 of true infra-glacial origin, consisting largely of blunted and striated stones, which 

 could only have been transported by the ice as ground -moraine. No 'Nunatakkr' 

 occurred within the mer de glace near this place, and not a vestige of surface moraine 

 was visible." Holst, Drygalski, and Chamberlin have also made similar observations. 

 The stones and boulders of such a ground-moraine must therefore have an infra-glacial 

 origin ; they have been torn away from the rocky bed over which the glacier or ice- 

 sheet flows. 



Direct evidence is also obtainable to show that glaciers not only abrade their beds, 



