80 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



which form the wooded ridges on either side of the Platte Valley in that 

 region. It also occurs in the form of narrow dikes, cutting through the 

 Archean. On the west side of the range it forms many large bodies in the 

 Weber Grits, the most important of which is the laccolite body of Buckeye 

 Peak. These bodies in the northwestern part of the region pass into the 

 closely allied variety called Eagle River Porphyry, with which they doubt- 

 less connect, and which will be described in detail in a forthcoming report 

 on the Ten-Mile district. 



GRAY PORPHYRY. 



This rock, which occurs only in the immediate vicinity of Leadville, is 

 in its typical form apparently a decomposed Lincoln or Eagle River Por- 

 phyry. It has the same mineral composition and frequently the large ortho- 

 clase crystals that the former has, and can be traced as a continuous sheet 

 through transition forms into the typical variety of the latter. It is almost 

 invariably decomposed, and on or near the surface is generally a greenish- 

 gray rock, showing numerous crystals in a prominent earthy-looking ground- 

 mass; in the mines it is usually found bleached and often reduced to a 

 white pasty mass in which the outlines of former crystalline constituents 

 are but faintly traceable. It is of importance in connection with the ore 

 deposits, as where it has crossed the Blue Limestone it has often played the 

 same role with regard to them as the White Porphyry. 



As distinguished from the Lincoln Porphyry the microscope detects 

 traces of former hornblende in the rock and finds glass inclusions in the 

 quartz and numerous fluid inclusions in the feldspar. 



Occurrence. The main sheet of Gray Porphyry, the only body which is 

 distinguished by a distinct color on the Leadville map, occurs above the 

 main sheet of White Porphyry in the northern half of the area shown on 

 that map, and extends beyond it to Mount Zion. Other bodies which 

 belong without question to this variety, as well as those which are more 

 doubtful, have, for reasons to be given below, been included under the color 

 of "Other porphyries" on this map. The most important of these is a sheet 

 occurring in the Blue Limestone, cutting transversely upwards from its base 

 to the overlying White Porphyry. Among those which are doubtful are 



