IRON FAULT. 399 



Relation of Iron fault to ore bodies. — In the area Under Consideration the plane 

 of the Iron fauU has been cut in so many places as to render its tracing 

 practically continuous at its intersection with the contact. Its continuation 

 has also been traced through the porphyry above the contact to the surface. 

 Winzes have been sunk just north of the Main incline to a depth of 100 

 feet on the fault fissure, and from the McDonald shaft 65 feet, as shown in 

 Sections A and B. Examinations of these workings, and descriptions of 

 them where they were no longer accessible, render it very evident that 

 the faulting has been posterior not only to the intrusion of the porphyry, 

 but also to the deposition of the ore. 



It has been soberly maintained by some experts when testifying in 

 lawsuits that the faulting was previous to the eruption of the porphyry 

 and that the latter flowed down over the successive benches formed by the 

 faults, following their surfaces. A consideration of the general geological 

 structure of the region, where instances abound showing that porphyry 

 bodies and sedimentary beds were both folded and faulted together, should 

 be sufficient to show how untenable is such a theor}'; but a sufficient refu- 

 tation is found at this very point in the fact that the fault plane can be 

 traced up to the surface through the overlying porphyry. 



That the ore was originall3' deposited previous to the faulting is less 

 self-evident, since in places there is a certain amount of alteration of the 

 limestone adjoining the fault plane and since ore has been actually found in 

 the fault fissure, which often has a widtli of three feet or more and is filled 

 with a dark clayey mass, bearing a certain resemblance to vein material. 

 The alteration is only such as might have been expected from the ac- 

 tion of surface waters j^assing across the ends of the contact adjoining the 

 fault, and consists merely in a slight impregnation or replacement of the 

 limestone by oxides of iron and manganese. This action extends at most 

 only a few feet into the limestone and is confined to a region comparatively 

 near the siirfece. Had the original ore-bearing currents actually followed 

 the plane of the fault, ore deposition would have extended to a much greater 

 distance into the body of the limestone from the fault plane than from its 

 upper surface, inasmuch as far easier access to percolating waters would 

 have been afforded by the numerous bedding planes. 



