120 GEOLOGY AND PAI^^ONTOLOGY. 



OBSERVATIONS UPON THE IGNEOUS AND METAMOEPHIO EOCKS, AND ASSO- 

 CIATED MINEEALS, IN THE BOUNDAEY SURVEY COLLECTIONS. 



The preceding catalogue of specimens, with observations upon their character and geological 

 age, and a resume of their geographical distribution, may very properly be followed by some 

 notice of their relative positions in the series, and their correspondence with, or difference from, 

 others of the same age in other parts of the country. 



From time to time, and from various sources, we have learned that large areas of the central 

 portion of this continent are occupied by rocks of igneous or metamorphic character ; and that 

 the plains and valleys present geological formations of different and more recent periods. We 

 have also been made aware of the entire distinctness, in character and origin, as well as 

 geographical separation, of the great mountain chain of the Cordilleras, or Sierra Nevada of 

 the North, from the more easterly ranges of the Eocky Mountains. 



Physically, the great central mountain region, or Eocky Mountain chain, with its subordinate 

 rano-es, is clearly as distinct from the western chain, notwithstanding there may be numerous 

 isolated peaks and short broken ranges, which form a partial connexion between them. Still, 

 again, the Sierra Nevada and the coast range are recognized as geographically distinct. 



Geology has likewise proved these several mountain ranges to be of different origin and of 

 different age. The Cordilleras, or Sierra Nevada, and the subordinate ranges, or isolated 

 mountains dependent upon that stupendous chain, are all of the older metamorphic rocks, 

 consisting of stratified rocks of Palfeozoic age, silurian, devonian, and perhaps, to some extent, 

 of carboniferous strata, which have been changed from their original condition, and finally 

 elevated into mountains. The lithological character and mineral products are identical with 

 the rocks of the Appalachian chain, which form the great elevation from Canada and Nova 

 Scotia to Alabama, on the eastern side of our continent. Their lithological characters and 

 mineral products correspond likewise with rocks known to be of that age in other parts of the 

 world. 



The series of specimens in the boundary collections, and the specimens in other collections, 

 brought from this mountain range, exhibit all the varieties of mineral materials and differences 

 of aggregation presented in a series of the rocks of the Appalachian formations. The auriferous 

 gravel of California is derived from the quartz veins in the slates and other rocks of the Sierra 

 Nevada, as the auriferous gravels of Canada, Virginia, the Carolinas, and Georgia are derived 

 from the quartz veins of the Appalachian rocks. The auriferous quartz veins of California are 

 the same in character, in age, and origin, as those traversing the metamorphic rocks of the 

 great eastern chain from Canada to Georgia. 



There are even stronger, though more subtle, analogies between the rocks and minerals of 

 these widely distant mountain regions, when submitted to the researches of the chemist. 

 Eocks and compound minerals, while known by the same names, are often found, on careful 

 analysis, to possess different proportions of certain elements ; or they may in one case contain 

 an elementary mineral substance not known in the other. Now, even in this regard, the 

 researches of chemistry have proved that certain mineral products of the one mountain chain 

 are precisely similar to the same in the other. And we might go still further, and show that 

 the order of succession among beds of a certain character is the same in both mountain chains, 



