108 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



The eastern spur of Lincoln, a narrow straight ridge, being relatively 

 much lower than the northeastern or southeastern spurs, is covered only by 

 beds of the Cambrian formation, the White and Blue Limestones which 

 still cap the other spurs having been removed by erosion. Section B B, 

 Atlas Sheet VIII,' passes through this spur and shows its profile and geo- 

 logical structure as well as can be expressed on so small a scale. In addi- 

 tion to the normal eastern dip, the beds have also a decided inclination to 

 the south, so that the spur presents a perpendicular wall on the north 

 towards Lincoln amphitheater, with a shallow ravine on the south sepa- 

 rating it from the southeastern spur, the slope of the spur in that direction 

 corresponding nearly with the dip of the beds. This southern dip is the 

 relic of a lateral fold or slight corrugation produced by the forces of con- 

 traction acting in a northerly and southerly direction at right angles to the 

 major force. The Lincoln amphitheater is thus shown to have been cut 

 out of the axis of an anticlinal fold, and in the sedimentary beds still re- 

 maining on the northeast spur a slight inclination to the northward can still 

 be detected, showing that they formed the northern member of this subor- 

 dinate fold. 



The Cambrian quartzites which form the mass of the spur are of the 

 characteristic white saccharoidal variety, thinly and evenly bedded, and 

 contain a slight development of white limestone, which has been occasion- 

 ally observed elsewhere in this formation. At the eastern end of the spur 

 is a cliff of quartzite, just above timber-line, below which the beds assume a 

 steeper dip, so that the lower slopes are occupied by outcrops of succes- 

 sively higher horizons. At the foot of this cliff are several prospect holes, 

 following deposits of copper an'd iron pyrite near or in contact with a body 

 of decomposed quartz-porphyry. A sheet of Lincoln Porphyry, which may 

 be part of the same body, caps the spur above the cliff and is cut through 

 by what seems to be a dike of porphyrite. The porphyrite contains both 

 biotite and hornblende (the latter being, however, largely predominant) and 

 is more decomposed than porphyrite rocks generally, both these minerals 



' By an*error in proof-reading, the line of this section, as given on the map in blue (Atlas Sheet 

 VI), is partially wrong. It. should pass from the summit of Mount Cameron to that of Mount 

 Lincoln, and from there down the eastern spur, whereas on the map it passes directly from Mount 

 Cameron to the spur. 



