MEXICAN BOUNDARY LINE. 119 



The relative position of these cretaceous beds is precisely the same throughout Texas that it 

 is along the valley of the Mississippi river, in the States of Tennessee, Arkansas, Illinois, and 

 Missouri, where they rest upon the upper carboniferous strata. 



In an economical point of view, the most important results shown by this collection are the 

 almost constant association of metalliferous products at the junction of the igneous with the 

 metamorphic rocks of the carboniferous period, or perhaps sometimes with metamorphic rocks 

 of more ancient date. 



The collection of silver lead ores and copper ores, from different veins, with the associated 

 rocks, show that they are always near the junction of the igneous formations, and the superin- 

 cumbent more or less altered limestones. The metalliferous veins, it would appear, always 

 penetrate the limestones, which vary in character from gray or grayish blue granular beds with 

 fossils, to light colored or nearly white crystalline limestones. The specimens which can be 

 identified are clearly of carboniferous age, though some erratic specimens show that the older 

 Palfeozoic limestones may enter into this combination ; and possibly some of the cretaceoiis beds 

 have become so altered as to be undistinguishable from the older rocks, though we have yet had 

 no proof of extensive metamorphism in rocks of this age. 



These circumstances, nevertheless, do not affect the general inferences regarding the metal- 

 liferous character of the rocks at or near the junction of the two systems. The facts before us 

 warrant the conclusion that the conditions enumerated apply not only to the region actually 

 travelled over, but also to the highly metalliferous regions farther to the south in the same 

 range. These facts also suggest the importance of a more careful examination of this range 

 in its northern extension, which we already know to have the same geological constitution, but 

 which has scarcely been explored with a view to its economical resources. 



