160 F. N. Guild — Eruptive Rocks in Mexico. 



18,000 feet above sea level, and contain numerous craters in 

 various stages of decay. The rocks composing them are ande- 

 sites and basalts with the same characteristics and variations as 

 elsewhere in Mexico, the older outflows being of lighter color 

 and more crystalline texture, the newer showing vitreous and 

 darker colored types. These lavas are associated with large 

 quantities of other products of volcanic activity such as ashes, 

 pumiceous material, etc. 



The object of this paper is to describe the mode of occur- 

 rence of some of these rocks, especially those found in and 

 near the valley of Mexico, and to describe their mineral ogical 

 and chemical composition. 



Popocatepetl. 



One of the most beautiful views which greets the visitor 

 entering the valley of Mexico, is that of two lofty peaks situated 

 on its extreme southeastern border and reaching above the 

 line of perpetual snow. They are called in the Aztec tongue 

 Popocatepetl, the Smoking Mountain, and Ixtaccihuatl, the 

 White Woman. The former is a symmetrical cone-shaped 

 peak whose cap of dazzling white presents a strong contrast 

 to the dark pine forests of the lower slopes. Ixtaccihuatl is 

 less symmetrical and has an irregular elongated summit. 



It is said that the first ascent of Popocatepetl was made by 

 one of the captains of Cortes in 1521; one of the men is 

 reported to have been let down into the crater by a rope for 

 the purpose of collecting sulphur to make gunpowder for car- 

 rying on the conquest.* At the present time frequent ascents 

 are made and a well-equipped party experiences no serious 

 discomforts. 



The height of the peak above sea level is 17,876 feet, and it 

 has the form of a symmetrical cone obliquely truncated. Its 

 slopes may be divided into three parts, a lower area of vegeta- 

 tion reaching to about 12,000 feet, then steep slopes of soft 

 volcanic sand with occasional ridges of dark andesite up to 

 about 14,000 feet, and finally the snow cap reaching to the 

 crater. The latter is probably not very thick, since there are 

 no crevasses or other evidences of glacial movements. 



The crater is elliptical in shape with a maximum diameter 

 of 612 meters and a minimum diameter of 400 meters. f The 

 inner rim is made up of irregular ledges or blocks of black an- 

 desite, with a slope towards the crater of about 20 meters, and 

 then there is a perpendicular cliff varying from 80 to 75 meters 

 in height. At its base is a talus slope extending towards the 

 center of the crater, which contains a small pond formed by the 



* Prescott, Conquest of Mexico, p. 44, vol. ii. 



fAguilera y Ordonez, Expedicion Cientifica al Popocatepetl, Mexico, 1895. 



