188 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



2. Section across arch of Archean. 



' Debris slopes above cliff. 



Saecbaioiilal quartzite, white and thiu-becUled. (50 



Caiubriau < Saecliaroidal quartzite, like above,, but stained and 



discolored CO 



Coarse quartzite, witb fragments of feldspar 1 



121 



Aicbean Eed granite; upper beds very much decomposed; 



red feldspars turned yellow by hydration of iron 

 oxide ; 250 feet to base of cliffs. 



The White Limestone seems relatively thicker and the Blue Limestone 

 thinner than usual. The evidence of erosion on the sandstone underlying 

 the latter, which consists in hollows and ridges two or three feet in depth 

 or height filled by a limestone breccia, is important as indicating a land 

 surface at the close of the Silurian. Unfortunately this was the only point 

 at which it was detected, so that it cannot be said with certainty that the 

 land elevation at that time was very widespread, although the apparent 

 absence of Devonian beds is indirect evidence that it was, as is also the 

 great variation observed in the thickness of the upper member of the Silu- 

 rian, the Parting Quartzite. 



The White Porphyry above the Blue Limestone has a maximum thick- 

 ness of about fifty feet on the west point of Little Zion and rapidly wedges 

 out to the north and east It has the usual appearance of the normal rock, 

 but the fresh-looking hexagonal crystals of dark mica are rather more 

 abundant than usual, for which reason they were separated and analyzed, 

 and found to be muscovite instead of biotite, with which determination their 

 optical properties agree. (See Appendix B, Table I, Anal. II.) Above 

 this is the Gray Porphyry, which readil}' disintegrates and crumbles into 

 coarse sand, and therefore can be traced along the west slope of Mount 

 Zion by the gentle slope which it forms at the foot of the steeper slope of 

 Mount Zion Porphyry above it. It has here a thickness of about 100 to 

 1.50 feet, which increases to the northwaixl; it evidently connects with the 

 immense sheets of Eagle River Porphyi-y at the northern limits of the maj). 

 The Mount Zion Porphyr}^ has a thickness of about 800 feet on the crest 

 of the ridge, but rapidly wedges out to the north. In its upper part, near 



