324 COSMOS. 



interesting observations of Pieschel (March, 1854) le and H. 

 de Saussure. The rock of the peak of Orizaba, like that of the 

 volcano of Toluca 17 which I ascended, is composed of horn- 

 blende, oligoclase, and a little obsidian; whilst the funda- 

 mental 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 myste- 

 rious 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 consists of black, partially upraised lava- 

 blocks of a fearfully wild appearance, and only sparingly 

 coated here and there with lichens, contrasting with the yel- 

 lowish white coat of pumice-stone which covers everything 

 for a long distance round. The latter consists here of coarsely 

 fibrous fragments of two or three inches in diameter, in 

 which hornblende crystals sometimes lie. This coarser 

 pumice-stone sand, is different from the very finely granular 

 sand, which, near the rock el Frayle and at the limit of per- 

 petual snow, on the volcano Popocatepetl, renders the ascent 

 so dangerous, because, when it is set in motion on steep decli- 

 vities, the sand-mass, rolling down, threatens to overwhelm 

 everything. Whether this lava field of fragments (in 

 Spanish JUalpais, in Sicily Sciarra viva, in Iceland Odaada- 

 Ilraun,} is due to ancient lateral eruptions of Popocate- 

 petl, situated one above the other, or to the somewhat 

 rounded cone of Tetlijolo (Cerro del Corazon de Piedra) I 



16 Zeitschrift fur Allgemeine Erdkunde, Bd. iv, s. 398. 



1 ' 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. 



