DOfONS. ] 



GLENWOOD SPRINGS TO ASPEN. 



413 



tion. across the strike of easterly dipping Carboniferous, Triassic, Juras- 

 sic, and Cretaceous beds. That of Castle creek lias a north and south 

 course, and follows closely the line of a great fault and reversed fold, 

 which brings Cambrian, Silurian, and Carboniferous beds up into juxta- 

 position with the red sandstones of the Trias. The reversed Ibid 

 structure is well seen in a little hill on the east side of the Roaring 

 Fork valley at the mouth of Castle creek, where the Jura-Trias beds 

 rest upon the Dakota Cretaceous, as shown in Fig. 24, copied from 

 Holmes. ° 



Immediately beyond Castle creek, in the angle between it and the 

 Roaring Fork, lies Aspen mountain, a steep sharp ridge rising 2,000 

 feet (010 in.) above the valley, made up of faulted Archean, Cambrian, 

 Silurian, and Carboniferous rocks. 



The pretty mining town of Aspen, the second in importance in the 

 State, lies in the valley of Roaring Fork at the east base of Aspen 

 mountain. It is built upon a Hood plain of the valley where it emerges 

 from the Archean rocks of the Sawatch into the upturned Paleozoic 

 beds which rest upon their Hanks. These beds strike diagonally across 



Fui. U, — Reversed fold and fault on Roaring Fork near Aspen. Oolo. 



the valley in a northeast direction, dipping 46° to 50° to the north- 

 west. The narrowest portion of the valley is at the upper end of the 

 town, where the harder Cambrian (piartzites project on either side, 

 forming a sort of gateway into the Archean area beyond. Below this 

 gateway the valley widens rapidly, and moraine material covers the 

 lower slopes of the hills to an elevation of 1,000 feet, forming a broad 

 bench on the- east side of the valley. 



The ores are sulphides, arsenides, and anfimonides of silver, and sul- 

 phides of lead and iron, with their decomposition products. Barite is 

 a common gangue material. They occur as a replacement of the Paleo- 

 zoic limestones, the greater pari being found in those of the Lower Car- 

 boniferous horizon, which are in great measure dolomiti/ed in the 

 \ icinity of the ore bodies. Immediately above this limestone occurs a 

 considerable thickness of black shales, in which is an intrusive sheet 

 of diorite, generally decomposed. 



The complicated geological structure of the region is the result of 

 extreme compression of sedimentary beds against an unyielding Archean 

 mass, which has thrust the beds into nionoclinal and reversed folds, 



