1 10 GEOLOGY AND MINING INDUSTRY OF LEADVILLB. 



The upper surface of the northeastern and southeastern spurs of Lin- 

 coln, respectively, is mainly formed of beds of Blue Limestone, which have 

 been opened by innumerable prospect-holes and several considerable mines 

 on either spur. On the steep cliff faces towards the Platte cafion and the 

 Cameron amphitheater, respectively, the limits of this formation and those 

 which underlie it can be distinctly traced. On the more rounded interior 

 slopes debris of Lincoln Porphyry obscure very largely the actual rock 

 surface. For this reason and also owing to the small scale of the map, the 

 outlines of the formations there indicated are somewhat generalized. 



The sharp summit of Lincoln itself is made up of a mass of typical Lin- 

 coln Porphyry, projecting boldly above the sedimentary beds and noticeable 

 for its vertical cleavage planes, producing a columnar structure which is best 

 seen on its steep south face. Lincoln Porphyry is also found for a consid- 

 erable distance down the east spur, and with it are associated shales and 

 grits belonging to the Weber Shale formation. The short, sharp ridge 

 directly west of the summit of Lincoln, and between it and the saddle that 

 separates Mount Lincoln from Mount Cameron, is also composed of a series 

 of beds which evidently belong to this horizon. They dip somewhat 

 sharply to the east and consist of greenish, yellowish, and reddish shales and 

 of micaceous quartzites, with a bed of black shale near the top, comprising 

 in all a thickness of about two hundred and forty feet. Below this is a bed 

 of Lincoln Porphyry, evidently interstratified, while on the saddle itself are 

 outcroppings of Blue Limestone. A deserted mine on this saddle, known 

 as the Present Help, the highest mine probably in the United States, is ap- 

 parently at or near the contact of the Blue Limestone with the overlying 

 porphyry ; its workings had been abandoned and were inaccessible. The 

 intense metamorphism shown in all the sedimentary beds near the summit 

 of Lincoln and the columnar structure of its porphyry render it probable 

 that the mass which forms the peak is directly above the channel through 

 which this rock was erupted. There is evidence also that from this channel 

 a sheet of the same rock was spread out over the surface of the Blue Lime- 

 stone, wWch was probably the determining cause of the great concentration 

 of mineral at this horizon. 



