OHAPTER II. 



IRON HILL GROUP. 



IRON HILL. 



General description. — Of the three principal groups of mines, that of Iron 

 Hill presents the simplest type, both in geological structure and in the 

 character of its ore deposits. It is that of a block of easterly-dipping beds, 

 with a fault on its western side, by whose displacement these beds have 

 been lifted in places about one thousand feet above their western continu- 

 ation, and in which the ore deposition has taken place at the upper surface 

 of the limestone bed, along its contact with the overlying porphyry, and 

 extending down at times into the mass of the limestone. This simple type 

 obtains only on the south end of Iron Hill, and even then in a somewhat 

 modified form, the north presenting, as will be seen later, the extreme of 

 complication. 



The area represented on the Iron Hill and North Iron Hill maps forms 

 topographically one continuous ridge. The map has been printed on two 

 sheets, partly because of its otherwise cumbersome size and partly because 

 the geological character of the opposite ends of the hill is very different. 



The Iron Hill map includes all of Iron Hill except its northern por- 

 tion, together with a part of Dome or Rock Hill, the spur which lies be- 

 tween California and Iowa gulches. It thus takes in all the mines belonging 

 to the Iron Silver Mining Company, to the La Plata Mining and Smelting 

 Company, and to the Silver Cord Combination, which represent the prin- 

 cipal developments outside the Adelaide- Argentine group in this portion 

 of the Leadville region. 



Iron Hill and its companion, Carbonate Hill, are flat-topped bosses or 

 shoulders, on the main spur of the Mosquito Range between California and 

 Evans gulches, whose form was evidently due originally to the displace- 



