142 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



London mine, then bends more to the southward across London Hill. Under 

 Mosquito Peak the beds lie in a shallow synclinal, with the Blue Limestone 

 rising up gently to the eastward against the line of the fault. On the south- 

 east slope of this peak the limestone forms a cliff wall, rising abruptly above 

 the granite on the other side of the fault, thus affording another illustration 

 of the fact that flat beds resist erosion more from the fact of their horizon- 

 tality than from any greater resisting power of the materials which com- 

 pose them. Half way between Mosquito Peak and London Hill, near the 

 New York mine, a thin bed of White Porphyry is found at the base of the 

 cliff under the limestone; the outcrops of the formations cannot be traced 

 continuously to London Hill, as its lower- slopes are covered by a great 

 thickness of debris. 



The London mine at the time of visit was opened by two tunnels, one 

 above the other, a short distance west of the line of the fault. The lower 

 tunnel, at the base of the hill, after passing through a great thickness of 

 debris, consisting of large rock fragments frozen into so solid a mass as to 

 require blasting, follows the stratification planes of nearly vertical beds of 

 light-colored limestone, whose strike is a little more to the west of north 

 than the direction of the fault plane. The dip of the strata is a little wot 

 of the vertical. Between the beds of limestone is a compact White Por- 

 phyry, which can in the mine hardly be distinguished from the limestone, 

 especially as it effervesces with acid ; it contains, however, occasional dark 

 flakes of mica, and chemical tests placed its character beyond a doubt, 

 though it contains a percentage of soluble matter, mainly carbonate of lime 

 with a little magnesia (10 per cent, in the specimen tested), which is too 

 high to have come from the decomposition of feldspar alone, and must, 

 therefore, be supposed to be an infiltration from the inclosing limestone.*. 

 The limestones adjoining the porphyry to the east are very light colored and 

 contain over 10 per cent, of silica, which is about the normal percentage of 

 the upper part of the White Limestone. As the ore deposits follow the 

 stratification planes, not much exploration has been done across the strata, 

 and owing to the metamorphosed condition of the rocks exact determina- 

 tions of horizon were not practicable. It may be assumed, however, that 

 the ore deposits of the London mine occur in the upper part, if not at the 



