TRUE VOLCANOES. 241 



The volcano Cotojmxi:* 18,881 feet, according to Bouguer. 



The volcano Sahama\ (Bolivia) : 22,354 feet, according to Pentland. 



The volcano with which the fifth group ends is more than 

 twice as high as ^Etna, and five times and a half as high as 

 Vesuvius. The scale of volcanoes that I have suggested, 

 starting from the lowly Maars (mine-craters without a 

 raised frame-work, which have cast forth olivin bombs sur- 

 rounded by half-fused fragments of slate) and ascending to 

 the still burning Sahama 22,354 feet in height, has shown 

 us that there is no necessary connection between the maxi- 

 mum of elevation, the smaller amount of the volcanic activi- 

 ty and the nature of the visible species of rock. Observa- 

 tions confined to single countries may readily lead us to er- 

 roneous conclusions. For example, in the part of Mexico 

 which lies in the torrid zone, all the snow-covered mount- 

 ains that is to say, the culminating points of the whole 

 country are certainly volcanoes ; and this is also usually 

 the case in the Cordilleras of Quito, if the dome-shaped 

 trachytic mountains, not opened at the summit (Chimborazo 

 and Corazon), are to be associated with volcanoes; on the 

 other hand, in the eastern chain of the Bolivian Andes the 

 highest mountains are entirely non-volcanic. The Nevados 



* Boussingault, accompanied by the talented Colonel Hall, has near- 

 ly reached the summit of Cotopaxi. He attained, according to bar- 

 ometrical measurement, to an altitude of 5746 metres, or 18,855 feet. 

 There was only a small space between him and the margin of the 

 crater, but the great looseness of the snow prevented his ascending 

 farther. Perhaps Bouguer's statement of altitude is rather too small, 

 as his complicated trigonometrical calculation depends upon the hy- 

 pothesis as to the elevation of the city of Quito. 



f The Sahama, which Pentland (Annuaire du Bureau des Longi- 

 tudes, 1830, p. 321) distinctly calls an active volcano, is situated, ac- 

 cording to his new map of the Vale of Titicaca (1848), to the east- 

 ward of Arica, in the western Cordillera. It is 928 feet higher than 

 Chimborazo, and the relative height of the lowest Japanese volcano 

 Cosima to the Sahama is as 1 to 30. I have hesitated in placing tho 

 Chilian Aconcagua, which, stated by Fitzroy in 1835 at 23,204 feet, 

 is, according to Pentland's correction, 23,911 feet, and according to 

 the most recent measurement (1845) of Captain Kellet of the frigate 

 Herald, 23,004 feet, in the fifth group, because, from the contradictory 

 opinions of Miers ( Voyage to Chili, vol. i., p. 283) and Charles Dar- 

 win (Journal of Researches into the Geology and Natural History of the 

 Various Countries visited by the Beagle, 2d ed., p. 291), it remains 

 doubtful whether this colossal mountain is a still ignited volcano. 

 Mrs. Somerville, Pentland, and Gilliss (Naval Astr. Exped., vol. i., p. 

 126) also deny its activity. Darwin says : "I\vas surprised at hear- 

 ing that the Aconcagua was in action the same night (15th January, 

 1835), because this mountain most rarely shows any sign of action." 

 VOL. V. L 



