178 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



exposure to the air. The contrast of the g-littering yellow of the pyrite 

 with its dull-black background of shale is extremely beautiful. This bed 

 of black shale represents the base of the Weber Shale formation. From 

 it were obtained the following forms : 



Discina MeeM. 

 Orthis carbonaria. 

 Chonetes granulifera. 



Streptorhynchns crass%(s. 

 Aviculopecten rectilaterarius. 



Below the black shales is the main sheet of White Porphyry in consid- 

 erable thickness, succeeded by the Blue Limestone which forms the eastern 

 edge of the spur or shoulder, while the White Limestone and underlying 

 quartzite can be traced along the steep slopes below. The series is here, 

 therefore, complete from the Lower Quartzite up to the Upper Coal Meas- 

 ure ; and, even had the fossils obtained in the latter not been found, the ex- 

 istence of such considerable thicknesses of limestone above the Weber 

 Grits would have been enough to determine their horizon. The gradual 

 passage observed from the shallow dip of 20° to the vertical dip adjoining 

 the fault is proved b}^ actual observation and furnishes an analogy for the 

 vertical dips already observed at the London fault. The fault line itself is 

 exposed in a tunnel and is exceptionally distinct on the ridge, its direction 

 being here N. 25° to 30° W. ; the adjoining rock on the east is a coarse- 

 grained granite and on the west shales and grits. Where opened, the fault 

 shows slickensides and a considerable development of clay selvage, with a 

 fine breccia of very dark color, the result; of friction. In the granite ad- 

 joining the fault there is visible decomposition for some ten or tifteen feet, 

 consisting in a partial kaolinization of the feldspars and a hydration of what- 

 ever oxides of iron it contains, which is evidentlj' due to the action of 

 waters which have followed the plane of the fault. 



In the basin at the head of the north fork f>f Weston's gulch, only 

 a few feet east of the line of the fault and apparently parallel with it, is a 

 vein of quartz in gi-anite, some six or eight feet in thickness, which can be 

 traced up the wall of the ridge. In the first saddle of the ridge above Empire 

 Hill is a dike of White Porphyry about twenty feet thick, in the vicinity of 

 which the granite is decomposed in a manner similar to that near the Weston 

 i'ault. This saddle is on a line with the fault which runs between Sheridan 



