108 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DcC. 5, 



remark, already quoted, that the upper limit of erosion in the Alps 

 was between 7000 and 8000 feet above the sea. The following are 

 the places where I noted it. 



On crossing both the Scheideck and the Wangen Alp, the line of 

 erosion is well seen at about 7500 ; in both these cases this line may 

 be observed on the nummulitic, as well as on the older rocks. 



In the ranges of the Hochwang and the Casanna, between Chur 

 and Klosters, which consist entirely of flysch, a rough estimate gave 

 me about the same height of 7500 feet for the line of erosion ; and 

 in the Klausen Pass, between Linththal and Altdorf, I noticed the 

 line at about the same height on the Jurassic, nummulitic, and flysch 

 rocks. 



From the top of the Splugen the line may be very well traced 

 round all the neighbouring mountains ; it is about 500 feet above 

 the Pass, or 7500 feet above the sea: it nearly corresponds to the 

 upper limit of the stratified slate-rocks of the neighbourhood, the 

 geological age of which is undetermined. 



In the Lukmanier Pass (6338 feet) and in the Pass of the Ober- 

 alp from Andermatt to the head of the Rhine (6580 feet), the 

 erosion reached to so great a height above the Pass, that I could not 

 estimate its upper limit accurately : in both instances the line was 

 somewhat above the highest Jurassic rocks. 



On the sides of the upper and middle parts of the Lake of Como 

 only a few tips of the highest hills of gneiss rise above the line of 

 erosion : as the highest hardly reach 8000 feet, the line must be 

 about 7500 feet high. 



Third Line of ErosioUy about 4800 feet above the sea. — The two 

 Mythen behind the town of Schwyz, consisting of cretaceous limestone, 

 afford an excellent example of the contrast between the rough steep 

 peaks above the erosion and the rounded shoulders below : the pass 

 between the two hills is 4726 feet above the sea, and is a little below 

 the line of erosion, which may be taken at about 4800 feet. 



On the sides of the Rigi (nagelfluh) and Mount Pilatus (creta- 

 ceous), the erosion ends at about the same height : but I did not get 

 any accurate measurements. 



The hills on both sides of the Lake of Thun rise above a line of 

 erosion, which, judging from their height, must nearly correspond 

 with that on the Mythen. 



Many of the limestone hills round the foot of the Lake of Como 

 have their highest points a little above a line of erosion ; as only a 

 few of them rise a little above 5000 feet, so this line must here nearly 

 correspond with that on the north flank of the Alps at 4800 feet 

 above the sea. 



General results of the preceding observations. — I have thus 

 pointed out three equally distinct and level lines of erosion to be 

 traced on the sides of the Alps at about 9000, 7500, and 4800 

 English feet above the sea, marking three long periods of rest in the 

 elevation of this great group of mountains, at each of which only so 

 much of the mountains as rises above this respective line then stood 



