Z PEOCEEDINGS OF THE NATION" AL MUSEUM vol.71 



GEOLOGY OF THE MINERAL DISTRICT 



Figure 1 presents the principal geographical and geological fea- 

 tures of the area from which most of the minerals to be described 

 were obtained. It represents the geology as shown on the areal 

 sheet of the Anthracite-Crested Butte folio except that the several 

 Paleozoic formations are combined in one unit. Cement Creek and 

 Brush Creek are tributaries of Slate Kiver, which enters the Gun- 

 nison near Gunnison City. Taylor River is the principal northern 

 branch of the Gunnison, and the basin under the Sawtooth Range 

 is its extreme head. 



The Archean gneisses and schists of the northeastern corner of 

 the figured district are on the western border of a large area, the 

 dominant feature of which is the Sawatch Range, whose crest is some 

 10 miles to the eastward. 



Upturned against the pre-Cambrian mass is a series of five 

 Paleozoic formations. These are, in order of succession : 1, Sawatch 

 quartzite (Cambrian) ; 2, Yule limestone (Silurian) ; 3, Leadville 

 limestone (Carboniferous) ; 4, Weber formation (Carboniferous) ; 

 and 5, the Maroon conglomerate (Carboniferous). The four lower 

 formations are relatively thin, none exceeding 500 feet in thickness, 

 except locally. In contrast the Maroon conglomerate attains a 

 maximum thickness of about 2,500 feet, but only its lower part is 

 present in the area of the figure. A remnant of Jurassic and Cre- 

 taceous (Dakota) beds is represented as dipping northeasterly into 

 the mass of Hunters Hill. This was interpreted by Eldridge as due 

 to deposition against a bluff of Carboniferous sediments, but in view 

 of the complex structure referred to later an overthrust fault seems 

 a more probable explanation. 



The sedimentary rocks of the area of Figure 1 are penetrated by 

 three large intrusive bodies, one notable dike and several minor 

 ones, which are no doubt but arms of the large masses. The prin- 

 cipal body extends westward for 10 miles, and then turns north for 

 several miles into the heart of the Elk Mountains. A similar branch 

 runs north from the Sawtooth Range. This is the southeastern ex- 

 tremity of the mass, originally mapped by the Hayden survey, the 

 relations of which to the great fault-fold of the Elk Mountains have 

 long excited the interest of geologists. The other two main intru- 

 sives occur in contact in the Italian Peak group. They may all be 

 connected at some depth. 



In the Anthracite-Crested Butte folio I called the mass in South 

 Italian Mountain granite and the other two bodies diorite. Unhap- 

 pily, both names fail to indicate the characteristic association of 

 plagioclase and orthoclase; each present in important amount in 

 both rocks. They belong to the group intermediate as regards the 



