TRUE VOLCANOES. 305 



From the geognostic description here completed of the vol- 

 cano of Jorullo we will pass to the more eastern parts of 

 Central America (Anahuac). Unmistakable lava streams, 

 the principal mass of which is usually basaltic, have been 

 poured out by the peak of Orizaba, according to the most 

 recent interesting observations of Pieschel (March, 1854)* 

 and H. de Saussure. The rock of the peak of Orizaba, like 

 that of the volcano of Toluca,f which I ascended, is com- 

 posed of hornblende, oligoclase, and a little obsidian ; while 

 the fundamental mass of Popocatepetl is a Chimborazo rock, 

 composed of very small crystals of oligoclase and augite. At 

 the foot of the eastern slope of Popocatepetl, westward of the 

 town La Puebla de los Angeles, in the Llano de Tetimpa, where 

 I measured the base for the determination of the elevation 

 of the two great nevados (Popocatepetl and Iztaccihuatl) 

 which bound the valley of Mexico, I found, at a height of 

 7000 feet above the sea, an extensive and mysterious kind 

 of lava-field. It is called the Malpais (rough rubbish-field) 

 of Atlachayacatl, a low trachytic dome, on the declivity of 

 which the River Atlaco rises and runs at an elevation of 

 from 60 to 85 feet above the adjacent plain, from east to 

 west, and consequently at right angles to the volcanoes. 

 From the Indian village of San Nicolas de los Ranchos to 

 San Buenaventura, I calculated the length of the Malpais at 

 more than 19,200 feet, and its breadth at 6400 feet. It con- 

 sists of black, partially upraised lava-blocks, of a fearfully 

 wild appearance, and only sparingly coated here and there 

 with lichens, contrasting with the yellowish-white coat of 

 pumice-stone which covers every thing for a long distance 

 round. The latter consists here of coarsely fibrous fragments 

 of two or three inches in diameter, in which hornblende crys- 

 tals sometimes lie. This coarser pumice-stone sand is differ- 

 ent from the very finely granular sand which, near the rock 



1736: according to Dufrc'noy, Memoires pour servir a tine Description 

 Geologique de la France, t. iv., p. 272. All the genetic questions are 

 discussed very completely and with praiseworthy impartiality in the 

 9th edition of Sir Charles Lyell's Principles of Geology, 1853, p. 369. 

 Even Bouguer {Figure de la Terre, 17-19, p. lxvi.) Avas not disinclined 

 to the idea of the upheaval of the volcano of Pichincha. He says : 

 "It is not impossible that the rock, which is burned and black, may 

 have been elevated by the action of subterranean fire." See also 

 p. xci. 



* Zeitsclirifl fur Allgemeine Erdkiinde, bd, iv., s. 398. 



t For the more certain determination of the minerals of which the 

 Mexican volcanoes are composed, old and recent collections made by 

 myself and Pieschel have been compared. 



