118 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



Limeslone horizon. There are several bodies of Linoohi Porphyry, besides 

 the main sheet near the summit, which are not shown on the map, as time 

 did not admit a sufficiently detailed stud}- to determine their outlines or 

 whether the}' were remnants of this sheet or distinct bodies. The upper 

 part of the Blue Limestone on this spur seems to have been pai-ticularly 

 rich in black chert concretions, which now lie scattered over the surface of 

 the ground, and from which Prof Lakes obtained the following fossils : 



Spin/erina (sp. like S. Spergenemis) 

 Spiri/era Bockymontana. 

 Productus costatus. 

 Euomphalus (.sp. i). 



Athyris mibtilita. 

 Streptori/nchuH crassus.^ 

 Pleurophorus oblongus. 



These were mainly collected in a slight depression of the ridge, where 

 the overlying porphyry had been eroded off, and therefore must have 

 come from the upper part of the horizon. 



The lower Paleozoic beds are exposed in section at various points 

 along the steep western wall of this spur, which faces Buckskin gulch. 

 They were examined at two points. At the extreme southern end of the 

 S2:)ur, just above the town of Buckskin Joe, where the steeper eastern dip 

 of the formation comes in, several ore bodies have been discovered, and the 

 now abandoned mines (the Excelsior, in White Limestone, and the Cri- 

 terion, in Lower Quartzite) were once woi'ked. At the Criterion mine a 

 thickness of 150 feet of quartzites was measured between the Archean and 

 the first bed of White Limestone. The ore bodies are accumulated here 

 along vertical planes, running northeast and southwest, which seems to be 

 the direction of a dike of dark-green decomposed porphyrite, whose out- 

 crops are found in the ravine below the mine, near the contact with the 

 Archean. There is evidence also of a slight displacement along a plane 

 running northeast and southwest, whose ujjthrow is to the west. At the 

 Excelsior mine, which is about a quarter of a mile farther west, near the 

 point of the cliff in the angle of the gulch, the ore bodies follow similar 

 and nearly parallel planes. A section measured on the cliff near the mine 

 gave the following thicknesses, in descending series : 



