BNDLicH.] ACIDIC PLUTONIC ERUPTIVES.. 211 



•whicb is about northwest to southeast, the granites have been forced 

 upward, carrying before them the enormous mass of sedimentaries su- 

 periuiposed. By studying Mr. Holmes's admirable sections (Report, 

 1874) we will find that the eruptive granite reached the surface in great- 

 est quantities at such points where the vertical dimensions of the sedi- 

 mentary beds showed their smallest development. In other words, 

 at the i)oints or along the line of least resistance. A large number of 

 dikes traverse the sedimentary beds adjoining the trachytic areas. Many 

 of them are undoubtedly granitic; others appear to be trachytic. Of 

 these latter Dr. Peale says,* " Some of them would probably be proved 

 to be trachytic, but I am of the opinion that if they could be traced to 

 the granite they would be seen to grade imperceptibly into it." 



In miueralogical character the granites of the Elk Mountains approach 

 those of the Sangre de Cristo. The general appearance shows a white 

 or gray color. Orthoclase, oligoclase, quartz, and muscovite, with some 

 biotite, compose the rocks. Associated with the granites of the range 

 are some porphyries and diorites. I am inclined, however, to attach but 

 little importance to their presence, as they probably belong to the same 

 eruption that produced the elevation of the range. 



We have, in the Elk Mountains, an example of an eruptive range that 

 offers the most favorable opportunities for study. Not only do we find 

 the granites and sedimentary beds in direct contact, but we find younger 

 eruptive rocks in the region immediately adjacent. Although perhaps 

 not in absolute local connection with the granitic masses, these younger 

 eruptives stand in the most intimate causal relation. They belong to 

 the series of trachorheites and will be discussed below. 



From Mr. Holmes's sectious it appears that the Middle Cretaceous 

 shales have actively participated in the folding and plication of sedi- 

 mentary strata. From the Silurian up the regular succession of forma- 

 tions — as characteristic for Colorado — is represented in the Elk Mount- 

 ains. All these have been subjected, uniformly, to the most severe 

 disturbances. We can, therefore, from this evidence, place the elevation 

 of the range into a period very near the close of the Cretaceous forma- 

 tion. This, at the same time, agrees with the usual experience, which 

 claims for the greatest eruptive activity in Colorado a time near the 

 close of the Cretaceous, mostly, however, approaching rather the Tertiary 

 epoch. 



On Station 38 of 1873, south of the Gunnison, an occurrence was ob- 

 served that I am inclined to bring into connection with the eruptives 

 of the Elk Mountains. Geologically considered, station 38 may be re- 

 garded as the extreme southern extension of the Elk Mountain Eange. 

 The granite there is associated with the same sedimentary beds as in 

 the Elk Mountains.t We have, in that instance, an eruptive granite, 

 which has broken out through a great thickness of sedimentary strata, 

 has formed a high, prominent peak, and was succeeded by an eruption 

 of trachytic lava. The force which permitted the protrusion of this 

 granite succeeded in forming a quaquaversal arrangement in the sedi- 

 mentary strata. In lithological characteristics this granite agrees 

 closely with that of the Elk Mountains and of the Sangre de Cristo. 

 Its relative position to older formations admits of no doubt as to its 

 method of appearing. So far as could be determined, the time of this 

 . granitic eruption fell into the same period that is accepted for the eleva- 

 tion of the Elk Mountain Eange proper. Isolated occurrences of such 

 granites may be found, but they are of no considerable extent. 



* Eep. U. S. Geol. Surv., 1873, p. 2G1. 



tReiJ. U. S. Geol. Surv., 1873, pp. 337 and 340. 



