MAARS. 231 



Pelvoux, near Brianc,on, (12,905 feet) belongs, forms an am- 

 phitheatre of thirty-two geographical miles in circumference, 

 in the centre of which is situated the small village of la 

 Berarde. The steep walls of this circular space rise to a height 

 of more than 9600 feet. The circumvallation itself is gneiss , 

 all the interior is granite. 88 In the Swiss and Savoy Alps, the 

 same formation presents itself repeatedly in small dimensions. 

 The Grand-Plateau of Mont-Blanc, in which Bravais and 

 Martins encamped for several days, is a closed amphi- 

 theatre with a nearly flat bottom at an elevation of nearly 

 12,811 feet; from tne midst of which the colossal pyramid 

 of the summit rises. 89 The same upheaving forces produce 

 similar forms, although modified by the composition of the 

 different rocks. The annular and cauldron-like valleys (val- 

 leys of elevation), described by Hoffman, Buckland, Mur- 

 chison, and Thurmann, in the sedimentary rocks of the north 

 of Germany, in Herefordshire, and the Jura mountains of 

 Porrentruy, are also connected with the phenomena here 

 described, as well as, although with a less degree of ana- 

 logy, some elevated plains of the Cordilleras enclosed on all 

 sides by mountain masses, in which are situated the towns 

 of Caxamarca (9362 feet), Bogota (8729 feet), and Mexico 

 (7469 feet), and in the Himalayas the cauldron-like valley 

 of Caschmir (5819 feet). 



Less related to the craters of elevation than to the above 

 described simplest form of volcanic activity (the action from 

 mere fissures), are the numerous Maars amongst the extinct 

 volcanoes of the Eifel ; cauldron-like depressions in non- 

 volcanic-rock (Devonian slate), and surrounded by slightly 

 elevated margins, formed by themselves. " These are as 

 been twice measured trigonometrically ; its height, according to Reboul, 

 is 11,443 feet (3481 metres), and, according to Coraboeui, 11,167 feet 

 (3404 metres). It is, therefore, 1705' feet lower than Mont Pelvoux, in 

 the French Alps, near Brian9on. The next in height to the Pic de 

 Nethou in the Pyrenees, are the Pic Posets or Erist, and of the group 

 of the Marbore", the Montperdu, and the Cylindre. 



88 Mtmoire pour servir a la Description Geologique dc la France, t. ii. 

 p. 339. Upon " valleys of elevation" and " encircling ridges" in the 

 Silurian formation, see the admirable description of Sir Roderick Mur- 

 chison in " The Silurian System," pt. i, pp. 427 442. 



89 Bravais and Martins, Observ. faites au Sommet et au Grand 

 Plateau du Mont-Blanc, in the Annuaire Meteorol. de la France pow 

 1850, p. 131. 



