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summers have enabled me to extend the survey of the valley of Cha- 

 mouni beyond the limits to which my Map was originally confined. 

 I have also obtained a great number of approximate altitudes of all 

 the highest summits of the chain of Mont Blanc, from the extended 

 base which the distance from the Mont Breven to the Croix de Fle- 

 g^re (above 15,400 feet) has afforded me. But the results are as 

 yet only partially calculated. I have also made some additions to 

 our knowledge of the geography of the eastern part of the chain of 

 Mont Blanc, by examining the Glacier of La Tour in its whole ex- 

 tent, which proved the configuration of the mountains to be different 

 from what has been represented on all the maps and models which I ' 

 have seen. The Glaciers of Argentiere and La Tour are separated 

 throughout by a rocky ridge, but the Glaciers of La Tour and Trient 

 all but unite at their highest parts, and the main chain is prolonged 

 with scarcely a break in the north-east direction, sending off only a 

 spur towards the Col de Balme, which, perhaps from being the poli- 

 tical boundary of Savoy and Switzerland, has been represented gene- 

 rally on an exaggerated scale. What surprised me most, was the 

 great elevation of the axis of the chain at the head of the Glaciers of 

 La Tour and Trient. I found it barometrically to be 4044 feet above 

 the chalet of the Col de Balme, which, from five comparisons made 

 with the observatory at Geneva, is 7291 English feet, or 2220 

 metres above the sea, a result agreeing closely with the recent mea- 

 surement by M. Favre, which is 2222 metres. Adding this result 

 to the former, we obtain 11,335 English feet for the height of the 

 granitic axis at the lowest point between the Glaciers of La Tour 

 and Salena on the side of the Swiss Val Ferret. By a single direct 

 barometrical comparison with Geneva, I obtained 11,284 English 

 feet above the sea, or 140 feet higher than the Col du Geant. I 

 was successful in traversing the Glacier of Salena to Orsieres the 

 same day, a pass which has not before been described, and which has 

 this interest, in addition to the singular wildness of the scenery, that 

 it includes those regions of beautiful crystallized protogine, here in 

 sitUf which have been known to geologists hitherto chiefly from the 

 numerous moraines which they form in the valleys of Ferret and of 

 the Rhone, and especially the majority of the blocks of Monthey, 

 which have been derived, according to M. de Buch, entirely from this 

 region of the Alps." 



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