210 GEOLOGY AND MINING INDUSTKY OF LEADVILLE. 



Ivincr Paleozoic formations. Their scenerj^ is moreover of an imposing and 

 Alpine character that would hardly be expected from the somewhat tame 

 appearance of the immediate vicinity of the city itself For this reason a 

 view of the more picturesque and instructive of the two, that of Iowa am- 

 phitheater, has been chosen for the frontispiece of this volume. 



Frontispiece. — lowa amphitheater, as will be seen on the Mosquito map, 

 is a bowl-shaped depression some 2,500 feet deep, with three main branches 

 or subsidiary amphitheaters extending up to the north, northeast, and south 

 between the bounding peaks, Dyer Mountain, Mount Sherman, and Mount 

 Sheridan. The view given in the frontispiece is the reproduction of a not 

 altogether satisfactory photograph taken from a point on the north side of 

 Iowa gulch, at the foot of the steeper southern slope of East Ball Mountain. 

 The spur in the foreground, on the right, is a portion of the north slope of 

 West Sheridan, formed of Archean rocks capped by Lower Quartzite ; that 

 on the left is the south spur of East Ball mountain, along which runs the 

 Mosquito fault; while the background is formed by the west face of Mount 

 Sherman, an almost vertical wall 2,400 feet in height. On this wall the 

 np per 1,400 feet are occupied by the main sheet of White Porphyry, over- 

 lying the lower Paleozoic beds, in which are also several minor sheets of 

 the same rock, too small to be indicated except in a general way on the 

 Mosquito map ; and the base of the cliff is formed by Archean rocks. In 

 the view the horizontal lines of the stratified beds can be readily distin- 

 guished from the somewhat gothic forms of weathering of the great mass 

 of porphyry above, but the lower portions of tlie cliff are almost entirely 

 hidden beneath talus slopes of d6hns, through which only liere and there 

 projects a portion of the Archean granite. 



In the granite exposures on the south bank of Iowa Creek, a little 

 above the point from which the frontispiece view is taken, are the best 

 examples of glacier action on rock surfaces in this region. The granite 

 bosses here have a gentle slope to the east and are steep on the west, the 

 whole upper surface of the rock being beautifully polished, grooved, and 

 striated, and the lines being parallel with the direction of the gulch. These 



