104 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 5, 



level of the upper limit of the erosion, it would have produced a 

 marked indentation round the mountains, such as was under consi- 

 deration ; and as the mountains gradually rose out of the waters, 

 they would have had their flanks worn into rounded slopes ; and 

 occasionally again indented by another line of erosion, whenever 

 their elevation was arrested and the waves continued to beat for a 

 long period against their flanks at another level*. 



I had already observed several such lines of erosion in the Alps at 

 different levels, and I soon met with a complete confirmation of their 

 marking the levels of the ocean at their respective periods, in finding 

 that they corresponded in height with the levels of the action of water 

 shown in the excavation of many valleys. The inquiry now assumed a 

 new interest, and I regretted that, not having seen its full importance 

 at the outset, I had let slip many opportunities of making observations 

 which would have been valuable. For, if by such observations we can 

 mark the successive stages at which the mountains rested on their rising 

 out of the ocean, we may hope to throw light on some most difficult 

 problems of geological inquiry connected with the elevation of moun- 

 tain chains : we shall see how far that elevation was sudden or gra- 

 dual ; whether the mountains of one chain were raised up together, 

 or separately ; and whether the different parts of the chains were 

 equally elevated. I am far from undertaking to answer all these 

 inquiries at present : I shall be satisfied if, in calling attention to a 

 field of inquiry applicable to other countries as well as to the Alps, 

 I have illustrated the subject sufficiently to lead to a due appreciation 

 of its importancef . 



* See R. Chambers, 'Ancient Sea Margins,' pp. 183-189, describing the Lines of 

 Erosion on the Eildon Hills. 



t The views of M. Agassiz on this subject will be found in his notes " On the 

 Glacial Theory and its recent progress,'' in the Edinburgh New Phil. Journ. vol. 

 xxxiii. pp. 232, 233 ; they have also been published by M. Desor in the ' Biblio- 

 theque Univ. de Geneve' for March and April 1841, under the title "Surle niveau 

 des Roches polies et sur les consequences qu'on pent en tirer;" and again at p. 15 

 of an account of an ascent of the Shreckhorn in the 'Revue Suisse' for June 1843. 

 M. Agassiz states in the first-mentioned paper that " in Switzerland there exists a 

 limit at about 9000 French feet (9589 English feet) above which the summits are 

 no longer polished, but where the rugged peaks present a very strildng contrast 

 to the lower surfaces, which are polished or at least moutonnes. In the exterior 

 chains of the Alps, the polishing does not reach to a greater height than 6000 or 

 7000 feet (6393 or 7460 English feet). It cannot be doubted that this limit, 

 which is so well marked, indicates the level of the bed of ice at the epoch of its 

 greatest thickness." 



M. Desor informs us that on the sides of the Seidelhorn the rocks are polished 

 to the height of nearly 8000 feet (8524 English feet), being about 200 feet above 

 the present level of the Glacier of the Finsteraar ; from which he concludes that 

 the upper limit of the polished rocks indicates also the upper ancient limit of the 

 glacier. On the sides of the Schreckhorn the polished rocks rise, as you ascend 

 the valley, until they are lost under the snow at an elevation of nearly 9000 feet 

 (9589 English feet), which he regards as the highest limit of polished rocks. 



M. Desor's remarks appear to apply entirely to the polishing of the surface of 

 rocks by the friction of glaciers; with which subject I have nothing to do at pre- 

 sent. But M. Agassiz appears to confound togetha- the superficial polishing by 

 glaciers and the deep erosion which has altered the forms of the mountains and 



