PHYSICAL STRUCTURE OF MEXICO VOLCANIC MOUNTAINS. 11 



The geological structure or physiognomy of Mexico is peculiar. 

 The great Cordillera of the Andes, which traverses the whole of 

 South America, from its southernmost limit, is exceedingly de- 

 pressed at the Isthmus of Panama, where its gentle swells serve 

 merely to form a barrier between the union of the Pacific and At- 

 lantic. But, as soon as this massive chain enters the broader 

 portion of North America, it divides into two gigantic arms, to the 

 east and west along the shores of the Gulf and of the Pacific, which 

 support between them a continuous lofty platform, or series of table 

 lands, crossed, broken, and intersected by innumerable and abrupt 

 sierras, some of which rise to the height of seventeen thousand feet 

 above the level of the sea. This geological structure prevails 

 throughout the whole of Mexico, as now bounded ; for, at the Rio 

 Grande, the southern limit of Texas, the land sinks to comparative 

 levels, and affords channels for the numerous and important streams 

 with which, Louisiana, Florida and Texas are abundantly irrigated. 

 Whilst this is the case on the northern and eastern confines of 

 Mexico, the western portion is still traversed by the main body of 

 the gigantic Cordillera, which, penetrating California with its icy 

 peaks of the Sierra Nevada, passes onward to the north until its 

 rocky walls are lost, beyond Oregon, in the wilderness that bounds 

 the Frozen Sea. 1 



The reader who pictures to himself such a country will easily 

 understand that all temperatures are gained in Mexico on the same 

 parallel of latitude, — or that eternal heat and eternal frost are en- 

 countered in crossing the country in a straight line from Vera Cruz 

 to the Pacific coast. It is a country hanging on the two slopes of 

 a mountain, one of which descends to the Gulf and the other to the 

 Western Ocean ; and the traveller, in penetrating it, even by the 

 road usually traversed by public conveyances, must attain a height 

 of ten thousand six hundred and sixty feet, before he begins to 

 descend into the valley of Mexico, which is, still, seven thousand 

 five hundred and forty-eight feet above the level of the sea ! Thus 



1 The high table land of Mexico which we have described, is said to owe its pie- 

 sent form to the circumstance that an ancient system of valleys in a chain of gra- 

 nitic mountains, has been filled up to the height of many thousand feet with various 

 volcanic products. Five active volcanos traverse Mexico from west to east, — 

 Tuxtla, Orizaba, Popocatepetl, Jorullo, and Colima. Jorulla which is in the cen- 

 tre of the great platform is no less than one hundred and twenty miles from the 

 nearest ocean, which is an important circumstance, showing that proximity to the 

 sea is not a necessary condition although certainly a very general characteristic of 

 the position of active volcanos. If the line which connects these five volcanic vent*, 

 in Mexico be prolonged westerly, it cuts the volcanic group in the Pacific called 

 ths group of Revilla-Gigedo. — LyelPs Geology, American edition, vol. 1, p. 294. 



