228 REACTION OF THE INTERIOR OF THE EARTH 



covery, on the occasion of an igneous eruption, in 1804, 

 of the island-volcano of Umnack, then newly risen from 

 the sea, in the Aleutian Archipelago. At the great 

 eruption of Vesuvius on the 12th of August 1805, 

 which Gray-Lussac and I observed together, he perceived 

 from time to time a predominating bituminous smell in 

 the burning crater. I have here brought together these 

 few and hitherto little regarded facts, because they help 

 to confirm the close interconnection of all manifestations 

 of volcanic activity, linking salses and naphtha-springs 

 with actual volcanoes. 



Encircling ridges analogous to those of craters of 

 elevation, show themselves in kinds of rock very dif- 

 ferent from trachyte, basalt, and porphyritic schist; 

 for example, according to Elie de Beaumont, in the 

 granite of the French Alps. The Oisans mass of 

 mountains, to which the highest summit in France, ( 3H ) 

 Mont Pelvoux near Brianpon (12,905 feet), belongs, 

 forms a circus of thirty-two geographical miles in cir- 

 cumference, in the middle of which lies the little village 

 of La Berarde. The steep walls of the circus rise to 

 a height of 9600 feet. The circumvallation itself is 

 gneiss; all within it is granite. ( 312 ) In the Swiss and 

 Savoy Alps the same form is shown in several cases, 

 in smaller dimensions. The Grand Plateau of Mont 

 Blanc, on which Bravais and Martins encamped several 

 days, is a closed circus, or amphitheatre, 12,810 feet 

 high, with an almost even floor, out of which the great 

 summit-pyramid rises. ( 3I3 ) The same upheaving forces 

 produce similar forms, though modified by the compo- 

 sition of the rocks on which they have been exerted. 

 The valleys of elevation, described by Hoffmann, Buck- 



