186 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



through which it came up, somewhere under Prospect Mountain. It varies 

 mucli in external appearance : in its most unaltered form, as obtained at a 

 depth of 200 feet in the Hattie bore-hole, it resembles a fine-grained granite 

 or granite-porphyry, while in the extreme of alteration, as found in some of 

 the shafts on the southwest slope of Prospect Mountain, it is hardly to be 

 distinguished from decomposed White Porphyry. It differs microscopically 

 from the fine-grained granites by the absence of microcline and by the 

 presence of prismatic microlites of plagioclase with rounded ends, which 

 are particularly abundant in the quartz and orthoclase. 



The structure of the southern slopes of Prospect Mountain, which is 

 somewhat complicated, is described in detail in Chapter V, and the relations 

 of the White and Mount Zion Porphyries are shown on the map of Leadville 

 and vicinity, where they have distinct colors. The outcrops of the thin 

 sheet of the former and of the underlying Blue Limestone, which occur 

 along the East Arkansas Valley at the foot of Prospect Mountain, are only 

 proved by prospect holes, the actual rock surface being buried under debris 

 slopes. 



Mount Zion. — The mountain mass of Mount Zion and its southwestern 

 shoulder, known as Little Zion, xDresents a somewhat similar structure to 

 Prospect Mountain, of which it originally formed a part, and shows better 

 outcrops by which to trace its geological structure. Towards the valley of 

 the East Arkansas, on the southeast face of Little Zion, are fine cliif sec- 

 tions showing an arch of Archean, over which the Paleozoic beds and 

 included sheets of porphyry are folded, with a steep dip to the northeast 

 and a more gentle one to the southwest. Along the south face of Little Zion 

 the Blue Limestone outcrops can be distinctly traced, gradually descending 

 the hill with a southeast dip until, opposite the brewery in the Arkansas 

 Valley, they come dov/n to the level of the flood plain and furnish raw 

 material to several lime-kilns. At the western extremity of the Little Zion 

 Ridge, beyond the limits of the map and opposite the junction of the East 

 fork with the Tennessee fork of the Arkansas, is a little hill of granite, 

 which is remarkable as being the only place where direct evidence is 

 afforded of any considerable inequalities in the Paleozoic ocean bottom. 



