MEXICAN BOUNDARY LINE. 121 



and prove also, by dynamic and chemical laws, that it could not have taken place ia any other 

 order. 



The present occasion does not require the details of comparison between these two ranges of 

 the same age. Still, it is not a little interesting to know that two mountain chains, produced 

 from the metamorphism of series of strata of the same age, now form, the one the eastern, and 

 the other the western, outlines of our continent. The one has a direction from northeast to 

 southwest, and the other, almost at right angles, from the northwest to the southeast, 

 giving us the great breadth of continent at the north, and the narrow southern extremity. 



The coast range of mountains presents us with quite distinctive features in the specimens, 

 and we know from many sources that it consists of recent igneous rocks and metamorphic strata 

 of very modern age, the igneous products being chiefly greenstone or basalt, amygdaloid, and 

 materials of similar character. Further east, the Cordilleras offer a striking contrast in the 

 collections to those made along the route travelled from the coast of Texas to the westward. 

 From the coast to the Rio Grande, the specimens from the Limpia range, from the Sierra 

 Madre, and the Organ Mountains present no character similar to those from the Cordilleras. 

 The granites are all of different aspect, with glassy feldspar, occuring in connexion with known 

 volcanic products, as porphyry, greenstone, and mixtures of quartz, feldspar, and olivine, etc. 

 There are among these no granites assuming a gneissoid structure ; no granites with shorl, 

 tourmaline, or garnets ; no talcous (pholerite) slates, chloritic or mica slate rocks, as in the 

 Cordilleras. The lithological aspect of the two collections is at once conclusive of their different 

 age and origin. 



Whatever the Rocky Mountains may offer in other parts of their range, that passed over in 

 the boundary survey gives no indication of the occurrence of the older metamorphic rocks. 

 Indeed, the materials of purely igneous origin so largely preponderate, that the few metamor- 

 phic specimens aj^pear quite subordinate ; while the observations accompanying the igneous 

 specimens prove that they form nearly entire mountains which are crossed upon the route. 



We are aware that further to the north there are extensive mountains, which bear rather the 

 character of metamorphic than of igneous products ; but even these do not resemble the meta- 

 morphic rocks of the western chain. 



In the specimens from this range, we see the predominating influence of volcanic action, and 

 the result of the same action in the influence of heated waters holding silex in solution, by 

 which the more porous masses have been penetrated and become solid, or so changed in color 

 and condition that there is an almost infinite variety of these products of one prime source. 



16 M 



