MODE OF OCCURRENCE 489 



along definite horizons. Rogers * says the occurrence of the ores is 

 empirical, and bears no very close relation to the underlying rocks, 

 except in the Kishacoquillas valley, where he finds that the ores occur 

 over the anticlinal axes in the fissures produced by the flexure of the 

 limestones. The parallelism is shown to the best advantage where the 

 slate belts are most pronounced, as the slate or its residual clay de- 

 termines the location of the ore bodies. The accompanying map of the 

 South Mountain area in Cumberland county illustrates this feature to 

 better advantage than any other area; yet, while it may be noticed that 

 most of the pits occur along well defined belts, quite a number can not 

 be so arranged (see figure 5). These pits lie in the slate area, where 

 there is some intercalary limestone, but the main limestone belt of the 

 valley lies north of the area shown on the map. In the limestone valley 

 areas the parallelism is so subordinate that it has little significance, as 

 shown on the accompanying map of a portion of the Lehigh County ore 

 region (see figure 6). 



In the Cumberland County region the prospectors recognize the align- 

 ment of the ore deposits and take advantage of it in seeking new bodies 

 of ore, but this can not be followed very successful^ in the more purely 

 limestone areas. 



In portions of Nittany valley a heavy bed of sandstone in the axis 

 of the anticline restricts the ores to a comparatively narrow belt on either 

 side of the sand ridge, and concentrates them to a considerable extent 

 along the contact of the sandstone and limestone, but elsewhere in the 

 valley ore pits have been opened at many different horizons, ranging 

 from the oldest limestones exposed to the overlying Utica shales. 



The conclusion of the writer is that the ore deposits may occur at any 

 horizon in the limestones, but since a great many of the larger deposits 

 occur on beds of sand or clay, residual from sandstones and slates, and 

 since these intercalated beds are generally of considerable extent, there 

 is in such places a consequent alignment of the deposits, corresponding 

 to the strike of the rocks and sufficiently pronounced in a region of slate 

 beds to be useful in following up the ore bodies, but in the limestones 

 free from intercalated slates or sandstones there is very little regularity 

 about the occurrence of the ore. 



Original Source of the Iron 



The discussion of the origin of the limonite ores may be conveniently 

 divided into two heads — the original source of the iron and the mode of 

 its concentration. 



* Geology of Pennsylvania, 1858, vol. 1, p. 479. 



