434 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



from tlie prominence given to small crystals of feldspar and the oxidation 

 of the contained iron, so that it is not to be distinguished from the average 

 Mottled Porphyry. This body can be traced but for a short distance in 

 either direction. It is probably an offshoot from the main underlying sheet, 

 and its position suggests a possible connection, or at least common origin, 

 ■with that found in the Morning Star ground, although the shape of the 

 latter, as shown in Section C, is more that of a sheet than of a dike. It 

 must be borne in mind, however, that none of these later intrusive and 

 cross-cutting sheets has the regularity of the normal dike as it is gener- 

 ally represented in geological text-books, and further that, as in the mines 

 they are seldom exposed in more than a few isolated points, their graphic 

 representation on the section is almost entirely ideal and subject to correc- 

 tion whenever further explorations furnish more facts in regard to them. 

 The White Porphyry in this shaft was found to be highly decomposed 

 throughout and so stained by iron oxides near the contact that the line of 

 the latter could not be accurately determined. In it, about fifteen feet above 

 the contact, was found a small body of ore, consisting of pyromorphite and 

 cerussite, with a little sulphide, filling the interstices of small blocks of 

 country rock. The rock contained over 80 per cent, of silica and may 

 have been an included fragment of impure quartzite belonging to the Weber 

 Shales. This was probably a secondarj' deposit. 



The occurrence of a second body of vein material below the Gray Por- 

 phyry is extremely interesting, as showing that replacement of the lime- 

 stone has taken place, at times, below this porphyry, as it has normally 

 below the White Porphyry. In this case the body is exceptionally rich in 

 manganese, being mainly the black iron of the miners. The jointing planes 

 are covered with a coating of fine crystals of pyrolusite. 



Upper shaft. — This shaft was sunk 290 feet through White Porphyry 

 before reaching the contact. Here the porphyry was hard and exception- 

 ally fresh, being what is locally known as "block porphyry"; moreover, 

 it contained minute crystals of pyrite, disseminated through its mass. The 

 occurrence of undecomposed pyrites in the porphyry is noteworthy in con- 

 nection with the fact that for a considerable distance to the east of the shaft 

 there is a close contact — that is, little or no replacement on the surface of the 



