446 GEOLOGY AND MINING INDUSTRY OF LEADVILLB. 



parallel to it on the northwest. There is, moreover, a general dip of the 

 folds and of the beds that compose them to the northeast. The structure 

 is further complicated by minor crumplings, which are shown by the wavy 

 outlines of the outcrops, but in which it is difficult to trace any regular law. 

 In addition to all this the development of porphyry has been exceptionally 

 great in this region. As already shown, the cross-cutting zone of White 

 Porphyry passes through the southwestern edge of the hill, so that there 

 is an underlying and an overlying body throughout the greater part of its 

 area ; and in the Amie ground the ore horizon is split up into three sheets, 

 each bounded by White Porphyry. Gray Porphyry is found as an overly- 

 ing sheet, as the continuation of the cross-cutting sheet of Carbonate Hill, 

 in a dike-like body, and in several small and apparently detached sheets. 



While on Iron and Carbonate Hills it is always possible to trace the 

 limits of the original body of Blue Limestone, which shows but a moderate 

 variation in thickness, on Fryer Hill it is difficult to explain the seemingly 

 enormous contraction that this bed has suffered through the action of replace- 

 ment. It is true that the intrusion of the porphyry mass has in many cases 

 split the original body into several isolated sheets, whose originally irregu- 

 lar shape would account for a certain variation in the thickness of the pres- 

 ent bodies of vein material. On the other hand, this explanation hardly 

 seems adequate for certain extreme instances, such as that in the Little 

 Chief mine, where within a distance of scarcely over one hundred feet the 

 iron body varies from a thickness of six feet up to ninety feet. It would 

 almost seem in such cases as if, in the plastic condition to which the pres- 

 ence of enormous quantities of surface waters had given rise, not only in 

 the ore bodies but also in the surrounding porphyry, an alternating thick- 

 ening and thinning of the body, perhaps already inaugurated by the shape 

 of the original limestone, had been very much increased by subsequent 

 compression within the mass. In other cases the finding of two bounding 

 quartzites, that which represents the Weber Shales and that forming the 

 Parting Quartzite, proves that there has been an absolute contraction due 

 to replacement. The average thickness of the iron body on Fryer Hill will 

 probably not exceed fifty feet, whereas, as has already been seen, the original 

 Blue Limestone often reaches 200 feet in thickness. 



