CARBONATE INCLINE. 419 



The section afforded by this incline is extremely interesting, as showing 

 the irregularities of the limestone surface caused by undulations or slight 

 flexures in the formation, and these are best seen in the graphic illustration 

 afforded on Atlas Sheet XXX, Section I, which has been very carefully 

 constructed from actual measurement. It will be seen by reference to this 

 section that the original surface of the limestone presents a general wavy 

 outline with one prominent fold, which, contrary to the general rule preva- 

 lent in the major flexures in the region, has its steeper side to the east. 

 The deciphering and reconstruction of the original folds in the formation is 

 a matter of some little delicacy, since the ore currents have eaten irregu- 

 larly into the mass of the limestone, and the porphyry itself is also some- 

 what altered and mineralized. It is evident, however, that the dividing line 

 between ore and limestone is one which must be entirely rejected for this 

 purpose. The parting between ore and porphyry, on the other hand, in 

 spite of occasional incursions of ore material into the mass of the porphyry, 

 is practically much more definite, and is that which has been used in deter- 

 mining the points in the original surface of contact. One prominent fact to 

 be observed in this section, and one which seems capable of a certain 

 amount of generalization, is that the main rich ore body is found adjoining 

 the crest of the prominent wave or fold in the limestone. It would seem 

 that along the line of this sharp fold, which may very possibly have been 

 accompanied by a slight displacement, there was an interruption in the ore 

 currents, as is further evidenced by the fact that beyond the fold on the 

 east side for a very considerable distance, indeed, to the extent of the 

 present developments, there has been no considerable deposition of pay ore, 

 the mineralized zone consisting of a most irregular replacement of the lime- 

 stone by what is known to the miners as black iron, a mixture of oxide of 

 manganese with clayey material, which passes by almost imperceptible 

 transition into coarsely-crystalline black limestone. In this lower part of 

 the incline is one of the most striking evidences of the fact that the ore 

 deposit is an actual replacement of a limestone in place, the walls of the 

 incline showing the clayey, ferro-manganiferous material penetrating irreg- 

 ularly into the limestone, now in thin, sheet-like bodies, following a cleav- 

 age or fracture plane and terminating in a point, and now replacing the 



