196 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



this arm is another .small area of quartzites, which seem to be entirely 

 inclosed by the Nevadite. Farther north, upon the eastern side of the 

 body, the strata are much disturbed and show varying strikes and dips, 

 and upon the northern slopes, within the Ten-Mile district, the strike of 

 the Weber beds is found to be east to west, with a northerly dip of 80 

 near the Nevadite, which lessens to 25 at a distance of half a mile. At 

 the northwest corner of the mass, just beyond the line of the map, the 

 eastern extension of the Eagle River Porphyry sheet shown in the synclinal 

 fold is found to be cut through by the Nevadite. Along the western con- 

 tact steep westerly dips are found in the sedimentary beds, and several thin 

 sheets of Eagle River Porphyry seem to be cut by it, but the relations are 

 not clear. A branch of Chalk Creek cuts deeply into the Nevadite and 

 testifies to the thickness of the body upon this side. 



Three of the smaller outlying bodies of rhyolite are apparent offshoots 

 from the Chalk Mountain mass, but the rock of Chalk Ridge is of another 

 type, allied closely to the body shown in the synclinal fold on the north 

 line of the map east of Ten-Mile Creek. This rock is fine grained, showing 

 only a few quartz grains and minute feldspars, and disintegrates readily 

 into a gravel-like mixture. The outcrop of Chalk Ridge is only a few hun- 

 dred yards in length, forming a sharp point between the mouth of Chalk Creek 

 and the Arkansas. Above it are sedimentary beds, dipping at a shallow angle 

 to the north and east and inclosing thin beds of Eagle River Porphyry. 



Opposite Chalk Ridge, on the west bank of the creek, a white rock is 

 disclosed in a little tunnel, which at first glance might be mistaken for rhy- 

 olite, but which on close examination proves to be simply altered Weber 

 sandstone, composed of limpid grains of quartz, white muscovite, and kao- 

 linized feldspar. In the gorge of Chalk Creek the first outcrops above the 

 Chalk Ridge are thinly-bedded limestones and shales. Higher up, where 

 the chasm is spanned by the railway bridge, are sandstones and quartz- 

 ites, with intrusive bodies of porphyry, generally interbedded, but also 

 crossing the strata. In the railroad cut on the eastern side of this gorge a 

 section is exposed, showing one of these intrusive masses crossing trans- 

 versely the beds and spreading out above. Figs. 2 and 3, Plate XIX, 

 represent sketches taken on the spot at the time when the cut was freshly 



