190 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



just beyond the extreme northwestern corner of the map, is the El Capitan 

 mine, which is of interest as being the only considerable ore deposit thus 

 far developed in this region at the Blue Limestone contact. In the Weber 

 Grits, which form the surface rocks from Piney Creek eastward across Chi- 

 cago Ridge to Chalk Mountain, as shown in Section A, are numerous bodies 

 of porphyry, which doubtless originate in an i'mmense laccolitic body whicli 

 occurs just north of the limits of the map, near the head of Eagle River, and 

 on a line with Chicago Ridge. 



East Arkansas Valley. — The flood-plaiu deposit, whicli forms benches on 

 either side of the alluvial bottom of the east fork of the Arkansas, extends 

 up a little distance beyond Howland post office, above the lower bend. 

 Under this no rock outcrops are visible. The south wall of the valley, 

 formed by the slopes of Pi'ospect Mountain, is mostly covered by debris, 

 but the north wall on the Mount Zion side has cliff-faces and abundant out- 

 crops. Above the arch of Archean, already described, the successive beds 

 of the lower Paleozoic series, the Gray and Mount Zion Porphyries, and 

 the thin bed of Weber Grits included in the latter descend into the valley 

 at a steep angle. The dip of the beds rapidly flattens out, however, as one 

 ascends the valley, and near the mouth of Buckeye gulch the Weber Grits 

 have become nearly horizontal. 



Between Ikickeye gulch and the bend of the valley below Howland an 

 important body of Lincoln Porphyry, with characteristic large orthoclase 

 feldspars, comes in, which can be traced up the valley wall for a distance 

 of two miles, apparently conformable with the bounding beds of Weber 

 Grits. A similar body exists on the east side of the valley, extending from 

 the mouth of Bird's Eye gulch up to a terminal moraine ridge half way 

 between Howland and Chalk ranch. It would seem that these two out- 

 crops are parts of the same great sheet of porphyry, though their connection 

 across the valley is not very distinct. The prevailing dip of the inclosing 

 sandstones is generally to the southeast. On the ridge between Ai-lcansas 

 Valley and Buckeye gulch this dip is quite pronounced and in places as 

 steep as 45°. IV"* wards its north end the eastern body forms a prominent 

 hill, called the Dome, with a steep face toward the valley, whicli ejiows a 

 tendency to columnar structure. The porphyry body is here much thicker 



