318 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 5, 



Bourg d'Oisans, to the head of the Glacier de la Pilatte. This 

 glacier occupies an extensive glen, guarded on one side by the huge 

 precipices of the Ailefroide (12,877 feet), on the other by a nameless 

 mass, the highest peak on which is probably about 12,000 feet. The 

 lowest point in the loop-like crete connecting these two portals is 

 about 10,800* feet above the sea. The exact height of the foot of the 

 glacier I do not know ; but probably 6800 feet is an estimate rather 

 above than below the truth ; for the ascent from La Berarde (5702 

 feet), 3^ miles down the vaUey (on map), is not great. Measuring 

 in the same way, it is just two miles from the Col du Sele to the foot 

 of the glacier (=10,560 feet). Neglecting the odd hundreds, which 

 we are justified in doing, as the lower part of the glacier is flat, we 

 have a fall of 4000 feet in 10,000 feet. From La Berarde to St. 

 Christophe, 6| miles (=35,640 feet), the fall is 879 feet, nearly 

 25 in 1000 feet. From the latter place to Venose (3363 feet), a 

 distance of about 3| miles, the fall is 1460 feet ; and from Venose to 

 the opening of the valley, perhaps about 4| miles, the fall must be 

 950 feet ; for the descent afterwards to Bourg d'Oisans (2380 feet) 

 is but slight. The fall of the valley between La Berarde and St. 

 Christophe is rather more than is given by the difference of the two 

 stations, as the latter place stands some height above the stream, 

 and this may a little affect the next measurement below. 



The result of this is, that the fall of the vaUey-bed during the 

 first two miles (measured on the map) is 40 in the 100, and 

 after that, for the remaining 17 miles of its course, its fall is 

 always considerably less than 10 in 100. Moreover, in that 

 uppermost portion, the slope very perceptibly increases as the 

 watershed is approached, so that the last few hundred feet, whether 

 of snow or rock, are of formidable steepness, the ascent to the 

 only accessible point (the Col du Sele) being a stiff bit of rock- 

 climbing. The same configuration is exhibited by most of the tri- 

 butary glens that join the main valley — by one, the Vallon de la 

 Bonne Pierre, leading up to the Col des Ecrins, to an even greater 

 degree. Three of these are of considerable sizef, and must in 

 former times have brought down immense tributary glaciers ; 

 they join the main valley at or below La Berarde ; and yet there is 

 not any marked deepening or widening of the vaUey caused by the 

 great increase of the ice-stream. 



Take, again, the glacier on the north side of the crete of the 

 Rateau (fig. 2) ; how could it have excavated that cliff which rises 

 fuU 2000 feet above it ? while, as if to make the puzzle greater, 

 within a mile to the west, begins the Glacier du Mont de Lans, which 

 covers a vast plateau, about 11,000 feet in height, sloping gently 

 to the north. Here the glacier recKnes like a white cloth upon a 

 table, protruding a few tongues of broken ice into two or three 

 comparatively shallow notches that have been worn out in its 

 northern edge. 



* The Col du Sel6 is 10,814 feet, mean of B. P. (Tuckett) and Bar. ( W, 

 Mathews) ; and several other parts of the ridge are of much the same height, 

 t The Vallons des Etan9ons, de la Muande, and de la Selle. 



