106 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



choidal fracture, a sherdy habit, and a clear ring under the hammer. It is 

 composed of feldspar, hornblende, and biotite, with a little quartz, and con- 

 tains from 60 to 63 per cent, of silica. To the naked eye no groundmass 

 is visible, although the crystalline ingredients are so minute (being gener- 

 ally less than l mm in size) that they cannot readily be recognized. A com- 

 mon variety (90) among the lower beds on the west and north is of coarser 

 grain and more decidedly green color, due doubtless to the presence of 

 chlorite. 



The most southern of the three transverse lines above mentioned runs 

 eastward from a little south of Alma, crosses several low forest-covered 

 ridges separated by small valleys, and shows only detached outcrops sep- 

 arated by frequent covered gaps. In this section only one body of por- 

 phyry and three distinct horizons of dolomitic limestone were found. The 

 beds, moreover, have a strike somewhat east of north and a dip of 25 or 

 more to the eastward, instead of a strike to the west of north and dips of 10 

 to 15, which prevail opposite the summit of Silverheels. The low ridge 

 bordering the Platte Valley is covered on the west side nearly to its sum- 

 mit by the lateral moraine of the Platte glacier, which must therefore at 

 one time have filled the valley to a level about 400 feet above its present 

 bottom. Lincoln Porphyry, a continuation of one of the bodies seen in 

 Beaver Ridge to the north, is disclosed by prospect holes. Various deep- 

 red sandstones are crossed, alternating with limestone and shales, but the 

 characteristic brick red of the Trias is first found at 'Crooked Creek, to the 

 east of Fairplay, in the forks of which is another important sheet of por- 

 phyry, probably the porphyritic trachyte of the Hayden map. This is in- 

 teresting as being different in appearance from any of the other porphyries 

 observed in the region and resembling that found in a railroad cut through 

 a Cretaceous ridge near Como. Nevertheless it does not possess the char- 

 acteristics of a Tertiary rock, unless a slightly rough feel may be consid- 

 ered such. It is of light-gray color and contains abundant porphyritically 

 disseminated crystals, mostly of white opaque feldspar, in a subordinated 

 groundmass. Two feldspars, hornblende, altered biotite, and quartz in large 

 but infrequent grains form its macroscopical constituents. Microscopically 

 the groundmass is seen to be evenly granular and the rock to be simply a 



