ON ITS EXTEKIOR. VOLCANOES. 387 



9. Mexico. 



The six Mexican volcanoes, Turtla*, Orizaba, Popo- 

 catepetl*, Toluca, Jorullo*, and Colima*, four of which 

 have been burning within historic times, have already 

 been enumerated ( 53 ), and their geologically remarkable 

 relative positions described. According to recent ex- 

 amination by Grustav Rose the formation of the rock of 

 Popocatepetl, the great volcano of Mexico, is the same 

 as that of Chimborazo, consisting, like it, of oligoclase 

 and augite. Even in the almost black beds of trachyte, 

 resembling pitchstone, oligoclase can be recognised in 

 very small crystals with oblique angles. The volcano of 

 Colima on the western side, on the shore of the Pacific, 

 belongs to the same Chimborazo and Teneriffe formation. 

 I have not myself seen Colima; but we owe to Pie- 

 schel( 531 ) (since the spring of 1855) a very instructive 

 review of the rocks collected by him, as well as interest- 

 ing geological notices upon the volcanoes of the entire 

 Mexican highland, all of which he has himself visited. 

 The volcano of Toluca, whose narrow highest summit of 

 difficult access (the Pico del Frayle), was ascended by 

 me on the 29th of September, 1803, and its height de- 

 termined barometrically at 15,168 feet, is very different 

 in mineralogical composition from the still active Popo- 

 catepetl, and the volcano of Colima, which is not to be 

 confounded with another higher, snow-clad summit of 

 the same name. The volcano of Toluca consists, like 

 the Peak of Orizaba, Puy de Chaumont in Auvergne, 

 and JEgina, of an association of oligoclase and horn- 

 blende. According to the above short statement we 

 perceive that, in the long series of volcanoes extending 

 cc 2 



