1 32 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



and Silurian beds above the Archean, tlie softer covered slopes on the top 

 of the ridge being underlaid by the Blue Limestone. The section from the 

 commencement of actual clilf slope downwards is as follows : 



Feet. 



Silurian ^ White Limestoue, not measured. 



\ Pori)li.vrite, 25 feet. 



• Quartzite and shales 50 



Porphyry, 20 feet. 



Quartzite ... 50 



„ , . Quartz iiornhvrv, 10 feet. 



Cambrian < ~* , . 



I Quartzite 25 



I Altered quartz-imrphyry, 15 feet. 



J Quartzite witli fine eoiij^lomerate at base . . 15 



L 140 



Archean Gneiss 



The upper intrusive bed is the normal hornblende-porphyrite, found 

 also on the opposite side of Mosquito gulch, and already described in the 

 Buckskin section. This bed, as will be observed in the sketch, is at the 

 base of the White Limestone on the right, and above this horizon in the 

 White Limestone on the left. It does not, however, break the continuity 

 of the sedimentary beds in the plane of this section as it passes from one 

 horizon to another, but it wedges out at one horizon and comes in again 

 a little further on, also in a wedge-shaped body, at a slightly higher horizon- 

 The rock of the second intrusive bed has also the external characters of 

 a porphyrite, and has been indicated as such in the sketch ; but micro- 

 scopical examination shows it to belong to the Green Porphyry type. The 

 third bed is a true quartz-porphyry, resembling the Lincoln Porphyry, but 

 without its large feldspars, and corresponds to the sheet in the Cambrian 

 on the Buckskin section. The lowest sheet is also a quartz-porphyry, but 

 so much altered that not much can be said as to its probable type. 



On the faulted-down ridge at the foot of the clitf the Green For- 

 j^lnrv forms the top rock, and the main bed of quartz-porphyry below 

 can be readily ti-aced ; but the lower one is less distinct. 



The chimney -like ravines which furrow the face of the cliff probably 

 follow, as on the Buckskin section, fracture or fault planes. In the one 

 which was examined, the left hand of the three from which the ddbris piles 

 descend in the sketch, there is a discrepancy of about six feet in the beds 

 on either side. 



