512 GEOLOGY AND MINING INDUSTRY OF LEADYILLE. 



down the gulch, however, opposite the Rock and Dome workings, it comes 

 to the surface again and is explored in the Nisi Prius and adjoining claims. 

 In the Nisi Prius very large bodies of highly manganiferous vein material 

 are found, wliich, tliougli not rich themselves, are generally considered to 

 indicate the proximity of a concentration of chloride of silver The tunnel 

 on this claim cut through a series of limestone ridges alternating with de- 

 pressions filled by vein material. That the ridges in this case do not repre- 

 sent actual folds in the formation is joroved by the fact that the stratifica- 

 tion lines can be seen to run horizontally across them. 



This is the westernmost point at which the contact has been reached 

 in Iowa gulch, and, while it may be found at no very great depth immedi- 

 ately west of Dome fault, the probabilities are that beyond that, as shown 

 l)y the cross-sections N, 0, and P, it is too deeply buried beneath Wash and 

 Lake beds to render its exploration advisable unless it were followed down 

 continuously on its western dip. The thickness of superincumbent detrital 

 material given in these sections is deduced from that found in the Coon 

 Valley and Black Cat and is probably a pretty close approximation to the 

 actual facts. The limits of the ore horizon to the westward, as indicated 

 bv the western outcrops of the Blue Limestone, are theoretical deductions, 

 and may vary somewhat from the facts : but the limit of error here is not a 

 very wide one, as actual outcrops of Archean on this strike-line are found 

 at a very short distance south of tlie boundary of the map. 



HEAD OF CALIFORNIA GULCH. 



On the north side of Printer Boy Hill and in the upper part of Cali- 

 fornia gulch the geological conditions ai-e extremely complicated, owing to 

 the number and variety of porphyry bodies and their rather unusual struct- 

 ural relations. The cross-cutting zone of White Porphyry passes through 

 here, as shown by the fact that on the north side of the gulch it comes in 

 contact with the Parting Quax'tzite and on the south with the Blue Lime- 

 stone. Abody of Pyritiferous Porphyry is also found cutting up across the 

 formation, being in the White Limestone at the bottom of the gulch, and 

 to the northward apparently joining the main body, which overlies both 

 White Porphyry and Blue Limestone. Li addition to these are several 



