TO JUNCTION OF GRAND AND GREEN RIVERS. 59 



and structure they seem to accord with that system, and in the present state of our 

 knowledge would naturally be included in it. In composition they generally exhibit 

 a marked difference from the ranges of the Rocky Mountain system. They are 

 composed in a much greater degree of purely eruptive rocks, whole ranges being formed 

 of trachytes, tufas, porphyries, amygdaloids, &c., in endless variety. Their granites 

 are generally white or gray. Metamorphic rocks are abundant among them, consisting 

 of gneiss, mica-slate, clay-slate, talcose-slate, and limestone, the latter highly meta- 

 morphosed and crystalline, forming white, blue, gray, and clouded marbles; so far as 

 observed, wholly destitute of fossils. They are rich in valuable minerals; large quanti- 

 ties of gold, silver, copper, lead, and iron having already been discovered in them. 

 The Black Mountains may be regarded as the most northerly, as well as the most 

 important, of these ranges which cross the Colorado. This chain crosses below the 

 mouth of the Virgen, and thence seems to trend away northwesterly toward Washoe. 

 It may be anticipated that the stores of metallic wealth so characteristic of these ranges 

 will be discovered in the unexplored region into which they are prolonged after crossing 

 the Colorado. 



The question of the relative age of the Colorado Mountains, and hence of the 

 Sierra Nevada and Rocky Mountains, is one of great geological interest, and yet one 

 on which at present we have almost no light. The prevailing mineralogical characters 

 of the two systems are, as has been said, considerably alike, and yet it remains to be 

 proven that in their southern prolongations they do not blend in one. 



I have suspected that the metamorphic limestone of the Sierra Nevada is of Car- 

 boniferous age, and that -it holds a position corresponding to that of the Rocky Moun- 

 tains. It is, however, so generally metamorphosed that all traces of fossils have disap- 

 peared. By reference to my report to Lieutenant Williamson, (P. R. R. Rep., vol. 

 VI, Greol. Rep., p. 27,) it will be seen that near Mount Shasta a Paleozoic limestone of 

 great thickness is exposed, which is highly fosilliferous. By Dr. Trask, who has exam- 

 ined it, it is regarded as Carboniferous. A further examination of this interesting locality 

 may, perhaps, give us the information which we so much desire. 



CERBAT MOUNTAINS. 



These mountains, with their associates, the Aquarius and Aztec ranges, are so 

 fully described in my first report on the geology of the Colorado country, that 

 nothing need now be said of them further than what is necessary to give them their 

 proper place in the wider view which later observations permit me to give of the Colo- 

 rado Basin; of this basin these mountains form the western margin. Against their 

 sides the older Paleozoic rocks seem to abut, and on their flanks the Carboniferous 

 rocks rest, just as on the slopes of the Rocky Mountains, three hundred miles eastward. 



In trend and composition these mountains seem to share the characters of the 

 Sierra Nevada and Rocky Mountain systems. Their granites are gray, and the sedi- 

 mentary rocks rest upon them nearly unchanged; the metamorphic slates and limestones 

 of the Colorado ranges not having been discovered. Their trend is slightly west of 

 north, nearly parallel with that of the Sierra Madre, their vis-a-vis. 



