FOUR-MILE AMPHITHEATER. 159 



Besides these two outcrops of limestone the only sedimentary beds ob- 

 served are a lenticular body of Weber Grits at the head of the amphithe- 

 ater on the south face of Mount Sherman. This body, which is several 

 hundred feet in length and thirty or forty feet in thickness, consists of shales 

 and sandstones, the former apparently somewhat baked and the latter 

 changed to quartzite. It extends to within a few feet of the top of the divid- 

 ing ridge between Four-Mile and Iowa amphitheaters, but does not outcrop 

 on the wall of the latter. 



The western slope of Mount Sherman, which forms the eastern wall of 

 the Iowa amphitheater and is shown in the background of the frontispiece 

 of this volume, consists, from the crest two-thirds way down, of a mass of 

 White Porphyry from 1,200 to 1,500 feet thick. Separating this from the 

 Archean in the bottom of the gulch are the lower Paleozoic series, whose 

 beds rise to the southward as one follows the wall and curve round the 

 west face of Mount Sheridan across the low saddle which separates it from 

 West Sheridan. The sharp crest of Mount Sheridan and its eastern slope 

 are covered with White Porphyry, as is also the little eminence south of it 

 on the main ridge, called Peerless Mountain. On the saddle between the 

 two the White Porphyry has been eroded off for a considerable distance 

 down the east slope, and certain rather silicious beds i esembling quartzite, 

 which here form the upper portion of the Blue Limestone, have been ex- 

 posed. South of Peerless Mountain the Blue Limestone is again exposed 

 on the surface of the crest, as far as the top of Horseshoe Mountain, and 

 also in a strip bordering the Horseshoe on the northeast. In this vicinity, 

 especially along the western face of Peerless Mountain, the upper portion 

 of the Blue Limestone shows evidence of considerable inetamorphic action. 

 Its outcrops are quite dark, and its upper part, as already mentioned, is very 

 silicious and resembles quartzite. It has also a slightly brecciated struct- 

 ure, and in certain places is very much stained with oxides of iron and man- 

 ganese. It is probable that this alteration is due to mineral waters, and is a 

 commencement of decomposition such as has gone on in Leadville itself, 

 though the amount of lead and silver ore as yet developed is comparatively 

 inconsiderable. The darker color is due doubtless to oxide of manganese, 

 and the silicification of the beds to percolating waters depositing granular 



