A SKETCH OF THE GEOLOGY OF MEXICO 385 



meters, continues without interruption from the high plains of 

 Texas and New Mexico to the valley of Toluca, which rises 

 against the flanks of the Nevada de Toluca, reaching an altitude 

 of about 2630 meters. This great meseta forms a geographic 

 unit of the first order. Breaks in the bounding mountains 

 afford easy communication with the coastal region and furnish 

 outlets for the drainage. It is a continuation of the Great Basin 

 region and has all the characteristics of that area. To the north 

 it widens and decreases in altitude ; to the south it narrows and 

 rises. At its vertex are situated the City of Mexico and the 

 two great volcanoes of the country. 



In Archsan time the southern, and a part of the western coast 

 of Mexico, rose above the waters and formed a series of islands 

 or perhaps a single strip of land which, as with the northern por- 

 tion of the continent, served as a point of initial deposition, and 

 around which has been laid down in successive geologic times 

 the beds which now make up North America. The rocks of 

 this period are numerous and present many variations between 

 different types. In the southern portion of Puebla, in Guerrero 

 and Oaxaca where the greater number of exposures occur, their 

 order of deposition was as follows : {a) Porphyritic gneiss sim- 

 ilar to augen-gneiss, losing its schistosity below and passing into 

 a species of granite; [b) Phyllite gneisses resting upon and 

 grading into the preceding beds; {c) Mica schists some- 

 what abundant, at certain points garnetiferous and comformable 

 with the rock below; {d) Phyllites, very argillaceous in their 

 upper portion, but with the clay gradually diminishing toward 

 the base and with concordant structural variation from stratiform 

 to schistose. These beds rest upon chlorite, sericite and amphi- 

 bole schists, which in turn cover the phyllite gneisses. 



Later than the deposition of the argillaceous phyllites and 

 before the end of the Palaeozoic, there were numerous eruptions 

 in the following order: (i) Gneissic granite, passing into a 

 porphyritic granite which cuts the mica schists without pene- 

 trating the phyllites. These rocks are shown in the northwestern 

 portion of the republic in the City of Caborca, Sonora. (2) 



