205 



tween the physiognomy of volcanoes and the 

 antiquity of their rocks. It is here sufficient to 

 observe in general, that the summits, which are 

 still subject to eruptions of the greatest vio- 

 lence, and at the nearest periods to each other, 

 are slender peaks of a conic form : that the moun- 

 tains with lengthened summits, and rugged with 

 small stony masses, are very old volcanoes, and 

 near being extinguished ; and that rounded tops 

 in the form of domes, or, shells, indicate those pro- 

 blematic porphyries, which are supposed to have 

 been heated in their primitive place, penetrated 

 by vapors, and forced up in a softened state, 

 without having ever flowed as real lithoidal lavas. 

 To the first * of these distinctions belong Coto- 

 paxi, the Peak of TenerifFe, and that of Orizava 

 in Mexico. The second-}- is common to Car- 

 gueirazo and Pichincha, in the province of 

 Quito ; to the volcano of Puracey, near Popa- 

 yan ; and perhaps also to Hecla, in Iceland. 

 The third % and last is found in the majestic 

 figure of Chimborazo, and, if it be permitted to 

 place by the side of this colossus a hill of Europe, 

 in the Great Sarcouy in Auvergne. 



In order to form a more exact idea of the ex- 

 ternal structure of volcanoes, it is important to 

 compare their perpendicular height with their 



* Picturesque Atlas, folio, PI. 10. 

 f Ibid, PI. 61. 

 + Ibid, PI. 1G. 



