PEALE ON ERUPTIVE MOUNTAINS IN COLORADO. 555 



in doubt as to the amount of elt^vatiou in the latter to be attributed to 

 the force which caused the u|)lit'tin^ of Mouut Guyot and Silverheels 

 Mountain. 



ELK MOUNTAINS. 



It is obviously impossible in the space at command here to give a com- 

 plete description of the Elk Mountains. For details, the reader is referred 

 to the Eeports of the Survey for 1873 and 1874. 



Topograpbically, the Elk Mountains might be considered merely as a 

 spur from the Sawatch Range, separated only by a high saddle. When 

 we study them closely, we find that geologically they are entirely distinct 

 The Elk Mountaiusconsistof four areas of igneous rock separated by syn- 

 clinal depressious filled with sedimeutaries. The eruptive force seems to 

 have acted along a line possessing- a trend northwest and southeast, thus 

 giving the mountains the character of a small range, from which the 

 beds di}) away to the northeast and southwest. Along this line there 

 are four areas of igneous rock, which seem to mark foci of greatest 

 force. Tliese areas are named from their prominent peaks as follows, 

 con)menciiig at the southeast: Italian, White Rock, Snow Mass, and 

 Sopris. Between them, the peaks composed of sedimentary beds rise to 

 an equal and in some instances to even a greater altitude than do those 

 of igneous rock. The eruptive material appears to have carried up the 

 sediujentary series, breaking and bursting through the strata. Subse- 

 quent erosion has removed an immense amount of material, and strata 

 that may have continued uninterruptedly across the mountains have 

 been removed. This erosion would naturally have the most effect over 

 the centers of greatest force, the disturbance of the strata breaking them 

 and rendering them more readily affected by the eroding agents. The 

 sedimentary rocks between the areas are much disturbed by a fault-fold 

 fully described in Mr. Holmes's report for 1874,* caused by the same 

 igneous rock which has not reached the surface. I shall now take up 

 the areas as named above. 



Italian area. — At Italian Peak a small mass of the eruptive material 

 has forced its way through the primordial rocks, carrying a portion of 

 them to the summit of the mountain. The quartzites at the base of the 

 series are highly metamorphosed, and irregular layers of the igneous rock 

 that have been thrust between the strata can be distinctly traced to the 

 junction with the main mass. On the east side of the ()eak, a small patch 

 of quartzites ha.s been caught between the eruptive rock and the adja- 

 cent metamori)hic Archaean rocks, as in the case near Mount Guyot, 

 already mentioned. 



In the Italian area, we have the only instance in the Elk Mountains of 



visible contact between the eru|)tive granite and the Archa'.in rocks, 



and the two are seen to be utterl^^ different. The rock of Italian Peak 



is fine-grained, and has all the appearance of granite. It is identical 



*Aiunial Report Uuited States Geological fSurvej' lor 1):<74, IH/ti, |)|). G"^-70. 



