EAST OF MOSQUITO FAULT. 211 



striation lines are particularly fine on the surface of the large feldspar crys- 

 tals, where, when closely examined, they are seen to resemble the parallel 

 lines of a steel engraving. 



Mosquito fault. — The average course of Mosquito iauh, which forms the 

 western boundary of this area, is magnetic north or north 15° east. From 

 the point where it branches off from the Weston fault, in the bed of Empire 

 gulch, it runs across Upper Long and Derry Ridge at the foot of the steep 

 face of West Sheridan, through Iowa gulch, along the west face of East 

 Ball Mountain, and through the narrow saddle on the ridge between West 

 Dyer Mountain and Little Ellen Hill into Evans amphitheater, which it 

 crosses diagonally, near if not actually through the shaft of the Best Friend 

 mine, to the foot of the zigzag road descending from Mosquito pass. Ow- 

 ing to the absence of shafts in the region, its location can only be determined 

 by actual rock outcrops, and where these are obscured by debris it may 

 vary a little one way or the other from that given on the map. Tts throw,, 

 which varies somewhat at different points, may be taken at an average of 

 4,000 feet. 



Minor faults. — Of the uiiuor or cross faults in this area only one, the 

 South Dyer fault, appears on the Leadville map. By its movement, which 

 was an upthrow to the north, a fragment of the Lower Quartzite beds, with 

 an included sheet of White Porphyry, has been left on the southwest spur 

 of East Ball Mountain, where it forms a shoulder half-way down the slope- 

 and is entirely surrounded by Arcbean outcrops. Beyond the line of the- 

 map it crosses the south foot of Dyer Mountain, where a dike of White 

 Porphyry cuts through the Archean on its probable continuation and joins 

 the Sheridan fault in the bed of Iowa gulch. The Sheridan fault runs at 

 rio-ht ano-les to the former in a southwest direction across the saddle between 

 Mount Sheridan and West Sheridan, and is supposed to join Weston fault 

 in the north head of Weston gulch. Its movem.ent is a slight upthrow to 

 the east, and the combined displacement of these two foults explains the 

 existence of a singular triangular-shaped mass of White Limestone and 

 Lower Quartzite at the entrance to the north branch of Iowa amphitheater, 

 in the very bed of the gulch. As the normal continuation of these beds is 

 found high up on the face of the surrounding mountains, it might seem at 



