114 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



which is also crossed by a nearly vertical dike. This vertical dike, as may 

 be seen on the left half of the sketch, can be traced from the Archean up 

 to the base of the Blue Limestone. It is from fifteen to twenty feet wide at 

 the bottom and branches at the top into five small arms, but does not spread 

 out between the strata. Its rock is a White Porphyry, which differs from any 

 of those observed elsewhere in carrying large orthoclase feldspars, sometimes 

 an inch in length. They are Carlsbad twins, and have a pinkish tinge like 

 those in the Lincoln Porphyry. Small rounded grains of quartz are also abun- 

 dant, but no trace of hornblende or biotite could be seen, either by the naked 

 eye or with the microscope. Under the microscope the feldspar is seen to be 

 partly plagioclase, and in the quartz are many small fluid inclusions. The in- 

 terbedded porphyry mass, like that on the south face of Lincoln, is prominent 

 by its dark color; but on examination it is seen to consist of two distinct 

 rocks, one of which seems to have pushed its way through the other after 

 it had been already spread out between the beds. The later rock is a Lin- 

 coln Porphyry, whose outlines can be distinguished from a little distance 

 by its peculiarity of weathering, its fragments showing larger surfaces than 

 that of the earlier rock. The earlier rock is of a light-green color, and 

 shows, to the naked eye, scarcely any distinguishable crystals, feldspars 

 being decomposed to a substance very like groundmass. Altered horn- 

 blende, a few biotite leaves, and an occasional quartz grain can be distin- 

 guished by the lens ; also, a few small specks of some metallic combination. 

 Under the microscope the groundmass resolves itself into a fully crystal- 

 line admixture of quartz, mica, and feldspar. Calcite is present in filmy 

 particles and occasionally in grains. The larger quartz crystals contain 

 fluid inclusions. The contact specimens of these two porphyries show a 

 blending of the characters of the two in the tendency to the formation of 

 large quartz and pink feldspar crystals in a base more like the older por- 

 phyry. As shown in the sketch, the Lincoln Porphyry throughout the 

 greater extent of the section is entirely included within the older mass. 

 Towards the eastern end, however, it forms a distinct bed above the other, 

 and each sends off a branch upward in a northeast direction across the strata, 

 forming nearly parallel dikes which meet at the surface of the ridge. These 



