F. N. Guild — Eruptive Rocks in Mexico. 159 



Aet. XVI. — Notes on Some Eruptive Rocks in Mexico ; by 

 F. N. Guild, University of Arizona. 



Introduction. — The central portion of Mexico is an elevated 

 mesa or plateau extending from the United States border to 

 the southern limits of the Republic. Its elevation varies from 

 about 3,000 feet on the United States line to nearly 8,000 feet 

 in the valley of Mexico. Its regularity is broken by numer- 

 ous mountain ranges, some of whose peaks reach above the 

 line of perpetual snow. The origin of this great central mesa 

 is one of the important geological questions in Mexico to-day. 

 Some authors* consider it to be due to great lateral fractures 

 and uplifts, while others, though admitting that no definite 

 explanation can be given until the region has been more thor- 

 oughly studied, are inclined towards different views. f 



The eruptive rocks on this plateau are represented by enor- 

 mous masses of rhyolite and andesite presenting great varia- 

 tions in texture, color, etc. Basalt is also abundantly devel- 

 oped but is of much less importance. The rhyolites and light- 

 colored porphyritic andesites are found quite uniformly 

 representing geologically older outflows, while the recent and 

 modern products of volcanic activity are practically all ande 

 sites of poorly developed crystalline texture and of basaltic 

 aspect. Excellent descriptions of many of these rocks may be 

 found in the publications of the Geological Institute of 

 Mexico.;): 



One of the most interesting places in the whole Republic is 

 the valley of Mexico whose interior consists of fertile sedi- 

 ments of great thickness derived from the disintegration of 

 the rocks along its border and from accumulations of volcanic 

 sand from many craters. It was formerly largely occupied by 

 extensive lakes ; Chalco, Xochimilco and Texcoco in the 

 south, San Cristobal, Xaltocan and Zumpango in the north. 

 Some of them have overflowed at times and inundated the city 

 of Mexico, causing great loss of life and property. Extensive 

 projects for draining them, begun by the Aztecs and carried 

 out by the Spaniards and Mexicans, have finally removed all 

 danger from this source. The valley is enclosed on all sides 

 by eruptive masses, some of which reach an altitude of nearly 



* Felix and Lent, Beitrage zur Geol. und Paleont. der Eep. Mex., Leipzig. 



f La Mesa Central de Mexico es un fenomeno totalmente secundario y no 

 debe referido £ grandes fracturas laterales, sino que se formaba por el rel- 

 lenamiento de los valles masaltos de la montana antigua, por masas de rocas 

 eruptivas, arenas volc&nicas y aluviones modernos. " Bose, Sobre el origen 

 de la Mesa Central Mexicana. r ' Bui. Num. 13, El Instituto Geol. de Mex., 

 1899, pp. 35-49. 



% E. Ordonez, Las Khyolitas de Mexico, Bui. del Ins. Geol. de Mex. Num. 

 14 y 15, 1900. Las Eocas Eruptivas del S. O. de la Cuenca de Mex. Bui. 

 Num. 2, 1895. 



Am. Jour. Sci.— Fourth Series, Vol. XXII, No. 128.— August, 1906. 

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