304: REACTION OF THE INTERIOR OF THE EARTH 



imitari. Iste terrae cumulus aperto veluti ore magnos 

 ignes evomuit, pumicesque et lapides cineresque." ( 439 ) 



From the geological description of the volcano of 



Jorullo which I have here completed, we pass to the 



more eastern part of Central Mexico. Unmistakable 



lava-currents,, of which the mass is chiefly basaltic, have 



been poured forth by the Peak of Orizaba, according to 



the most recent and interesting researches of Pieschel 



(March 1854)( 440 ) and H. de Saussure. The rock of 



the Peak of Orizaba, like that of the great volcano of 



Toluca( 441 ), which I ascended, is composed of hornblende, 



oligoclase, and some obsidian, whereas the fundamental 



mass of Popocatepetl is a Chimborazo rock composed 



of very small crystals of oligoclase and augite. At the 



foot of the eastern declivity of Popocatepetl, west of the 



town la Puebla de los Angeles, I found, on the Llano de 



Tetimpa (where I measured a base for determining 



the heights of the two great nevados which bound the 



valley of Mexico, Popocatepetl and Iztaccihuatl), an 



extensive and in some respects perplexing lava-field 



(or field of rough fragments of lava). It is called the 



Malpais of Atlachayacatl (the latter being the name 



of a low trachytic dome, on the side of which the Eio 



Atlaco takes its rise); it extends from east to west, 



or almost at right angles to the volcanoes, rising 



abruptly from about 64 to 85 feet above the adjacent 



plain. From the Indian village of San Nicholas de los 



Ranchos to San Buenaventura, I estimated the length of 



the Malpais at above 19,200 feet, and its breadth at 



about 6400 feet. On it blocks of black lava, only 



scantily overgrown here and there with lichen, of 



fantastic wildness of form and position, contrast with 



