irAK%iN-E.] GEOLOGY METAMOEPHIC ROCKS OP THE MOUNTAINS. 143 



occurs I suppose it difficult to tell, but whatever tlie conditions, whether 

 more or less heat, assisted more or less by aqueous action, they have 

 apparently been such that plasticity or liquidity has actually been pro- 

 duced in portions of these rocks, and probably to a considerable extent. 

 Such conditions, or a tendency to them, would certainly assist in pro- 

 ducing homogeneous or structureless masses of rock, by allowing freer 

 play of the particles in obeyance to chemical and other molecular forces, 

 and the large granite masses, indicating centers of greater metamorphism, 

 probably also indicate areas of once greater plasticity. While meta- 

 morphism alone has often left sharp lines of demarkation between dif- 

 ferently affected rock, there are also points where movements of the 

 plastic rock seem to have occurred 5 while, in tracing a line of schist 

 into a granite area, points may occur where the normal granitoid strata 

 regularly belonging to the series may gradually increase in number and 

 thickness, monopolizing the series and producing a normal metamorph- 

 ism ; or tongues of granite may invade the schists, as if an active meta- 

 morr)hism had proceeded outward from the granites, eating, as it were, 

 into the schiste, and absorbing first those beds by nature most readily 

 succumbing to the change, and leaving the intercalated masses less 

 changed. Yet the remnants of structure left in the granites still show 

 that no important movement has taken place in the mass, but that the 

 rock remains in situ, and is an indigenous granite. But, besides these 

 confusing appearances, lines of the granite sometimes appear as if actu- 

 ally injected or intruded among the schists, sometimes on their bedding, 

 but perhaps across it as eruptive veins. Indeed, there seemed cases 

 where, in approaching the same mass of granite from different points, 

 that at one all the appearances of a truly exotic and eruptive origin 

 might be found — abrupt lines of demarkation and veins ; while at another 

 point nearly all the steps of a gradual metamorphism and transition 

 from the schists beyond might be traced, while the remnants of struc- 

 ture through the mass itself would, in greater part, conform to the sur- 

 rounding system of folds, showing it as a whole to be an indigenous 

 mass. Two observers thus approaching such a mass would justly ren- 

 der different verdicts as to its nature, one ascribing to it a wholly erup- 

 tive origin, the other a clearly metamorphic character. A few minor 

 masses of granite did not show well-marked transitions from schists, 

 though in part the ends of the latter gradually, though yet abruptly, 

 merged into the granite, as if absorbed by it, the mass as a whole pre- 

 senting the character of an intrusive mass. There is no evidence what- 

 ever, however, to show that such masses have traveled far, ortiiatthey 

 might not have come from a short distance only, and have been derived 

 from rocks similar to those in which they are inclosed, or others of the 

 same series, for their likeness may be found at other points as true meta- 

 morphics. Penetrating various portions of the series are granitic, usu- 

 ally mostly feldspathic, veins, many of which probably extend long dis- 

 tances and appear to be of true eruptive character, while other granitic 

 veins, usually of very coarse bluish quartz and white cleavable feld- 

 spar, with sheets and large crystals of white mica, seem to be more 

 naturally referred to infiltration, or to be endogenous in character, like 

 many metalliferous veins, some of each kind showing layers of deposi- 

 tion or structure. Nearly all of the metalliferous deposits, for the pro- 

 fusion and richness of which Colorado is so justly celebrated, occur in 

 veins in this great system of metamorphic rocks, or in debris derived 

 from the same. The more noted of these were studied with care by Dr. 

 F. M. Endlich daring the early portion of the season, and his results will 

 be found as a i)ortiou of his report as geologist of the southern or San 



