100 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



by surface accumulations, either talus slopes or alluvial soil. In the rela- 

 tively wider valley of the Platte, however, about half a mile below the town 

 of Montgomery, a moraine ridge which crosses the valley once dammed up 

 a shallow lake basin, now a bit of meadow-laud; the present stream, which 

 drains this basin, exposes as it cuts through this ridge the quartzites and 

 shales of the Lower Quartzite formation and a considerable portion of the 

 overlying White Limestone, striking N. 15° E. and dipping 20° to the east. 



Hoosier pass ridge — On the slopcs of the Hoosicr pass ridge, just above 

 Montgomer}-, about one hundred feet of the Lower Quartzite are again ex- 

 posed in section, with two parallel intrusive sheets of porphyrite, the one lU, 

 the other 40 feet thick; the whole dipping 25° to 30° east, with a strike to 

 the west of north. This rock is the mica variety, having for its chief con- 

 stituents a white plagioclase feldspar, with a much altered biotite and a few 

 scattering quartz grains, in a dull-green groundmass. 



The general line of contact of the Cambrian is traceable along the 

 slope towai'ds the crest of the North Peak ridge, but distinct outcrops ai'e 

 first found again at the saddle between Montgomery and the Blue River, 

 over which a horse-trail leads. This saddle marks the outcrops of the Blue 

 Limestone, which consist of a dark iron-stained dolomite, weathering black 

 and caiTying thin seams of barite. On the east of the saddle its limits are 

 somewhat loosely defined by outcrops of blue shales, canying casts of 

 Zaphrenti^ and corals, which form a little knoll on the ridge, and may be 

 assumed to belong to the shale member of the Weber series. On the west 

 of the saddle, outcrops of the Blue and White Limestones extend to the 

 steeper slopes of the North Peak ridge, where their limits are defined by a 

 bed of green, fine-grained, silicious shale, impregnated with cubes of pyrite 

 which at times forms beds a foot in thickness. Only the Lower Quartzite 

 beds extend west of this on to the higher portion of the ridge. The work- 

 ings of the now abandoned North Star mine on the first shoulder of the 

 ridge have, as shown by the dump, passed through this quartzite into the 

 schists of the Archean. 



On the northeast face of the ridge, overlooking the valley of the west 

 fork of Blue River, is a small amphitheater with a little lake in its basin, 

 which the topography of the map shews but imperfectly. It is entirely in 



