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



feet across ; the soil formed at its bottom is very fertile and 

 these craters seem to be favorite places for cultivating corn. 



Next in line in this interesting group of volcanoes is Cerro 

 de San Nicolas, one of the smallest but most symmetrical of the 

 cones. It is about 350 ft. high, as found by barometric meas- 

 urements, and contains a small crater possibly 50 feet deep. 

 The material ejected from this crater both from a megascopic 

 and microscopic standpoint presents about the same character- 

 istics as in the last two cones. 



About three miles from San Nicolas is another low eleva- 

 tion called Cerro de Ixtapalapa whose crater has been com- 

 pletely removed by erosion. Near the summit the component 

 material is the same as that of the other craters of the group, 

 but its base is of compact lava which has spread out for some 

 distance forming gentle slopes. In this respect Cerro de Ixta- 

 palapa resembles Cerro de Catarina more than the other cones. 

 Under the microscopes this compact lava is more crystallized 

 than any of the others examined. Feldspar occurs in two dis- 

 tinct generations although no phenocrysts can be detected by 

 the naked eye. Orthorhombic pyroxene is abundantly devel- 

 oped in the form of isolated crystals and clusters. This rock 

 resembles quite closely those already described from the Sierra 

 de Guadalupe. It may be called a hypersthene andesite. 



Summary. 

 It will be seen from the foregoing descriptions that the 

 valley of Mexico and vicinity represent a petrographic 

 province in which the intermediate and basic types of rocks 

 are abundantly developed. In this respect it resembles regions 

 of recent volcanic activity in the western portion of the United 

 States and other parts of the world. The hypersthene andesite 

 from Popocatepetl and the Sierra de Guadalupe are very 

 similar to those described from Crater Lake, Ore.,* Mt. 

 Shaster, Cal.,f Buffalo Peak, Col. 4 and other well known local- 

 ities. Further the material ejected from Colima§ and other 

 active volcanoes in Mexico during their recent periods of 

 eruption seems to be of the same general mineralogical com- 

 position. The chemical composition is also similar, as may be 

 observed by comparing the analyses accompanying this paper 

 with those from the localities mentioned above. It is inter- 

 esting to observe the relation between the chemical composi- 

 tion of the older crystalline andesites as illustrated by the rock 

 from Chapul tepee, and the new T er, non-porphyritic, more vitre- 



* Diller and Pattern, The Geology and Petrography of Crater Lake National 

 Park. PP No. 3, U. S. Geol. Sur. 



f Diller, U. S. Geol. Sur. Bui. No. 150, p. 227. 



% Cross, U. S. Geol. Sur. Bui. No. 150, p. 224. 



§ Ordonez, Les Dernieres Eruptions du Volcan de Colima, Mexico, 1903. 

 Also a review by the writer in Geologisches Centralblatt, Bd. VII, No. 8. 



