ART. 18 MINERALS OF ITALIAN MOUNTAIN CROSS AND SHANNON 7 



As shown by the Hayden map, the intrusive of South Italian 

 Mountain extends about 3 miles to the southeast, forming a nearly 

 straight ridge. The great amount of talus and slide rock about 

 both intrusives effectually conceals contacts with the sedimentary 

 beds except in a few localities. At observed contact exposures the 

 intrusive was obviously crosscutting. 



The long dike crossing Hunters Hill is presumably an offshoot 

 from the conduit of the North Italian Mountain body, for its central 

 portion is very similar to the hornblendic contact zone facies of the 

 large mass. At its maximum width of 250 feet in Cement Creek 

 Valley there are dark lamprophyric contact zones 20 feet or more 

 wide on either side of this dike, bu they are inconspicuous in nar- 

 rower portions. 



OCCURRENCE OF THE MINERALS 



The minerals to be described by Mr. Shannon occur very abun- 

 dantly in a contact zone, of very variable width from place to place, 

 about the quartz monzonite intrusive masses above considered. Most 

 of them also occur in crystalline coatings on the walls of narrow 

 fissures in the intrusive rocks or perhaps filling such cracks com- 

 pletely. 



The most highly altered sediments, in which the minerals are 

 most perfectly and freely developed, are in the wedge-shaped area 

 caught between the two intrusive masses of the Italian Mountain 

 group. At and about the summit of the central peak where several 

 small dike offshoots penetrate the sediments the secondary minerals 

 are abundant and the original character of the strata entirely ob- 

 literated. The yellowish-green fluoriferous epidote and chlorine- 

 bearing mizzonite, of which Mr. Shamion quotes analyses by Eakins, 

 were found on the central peak of the group. 



Another exceptionally fine spot for collecting the minerals is 

 near the summit of North Italian Mountain on the west and north. 

 It was here that a carbonaceous shale of the Weber formation was 

 changed to a graphitic mass with garnet crystals, black from in- 

 cluded particles. 



The extensive alteration of strata about the larger and very irreg- 

 ular intrusive body results in the formation of secondary minerals 

 of good crystal habit in many places but far less commonly than in 

 Italian Mountain. But the presence of garnet, epidote, and pyroxene 

 in crystals attracting attention was noted in many places a mile or 

 more from the surface contact of the intrusive body. For most of 

 the contact zone in the Crested Butte quadrangle the Maroon con- 

 glomerate is the formation adjacent to the igneous rock. The strik- 

 ing red color of the beds, normal where they are distant from intru- 

 sive masses, gives way in the contact zone to somber purple or mottled 



