MODE OF OCCURRENCE 



485 



sandstones. The residual material consists of clay, sand, and chert, with 

 occasional fragments of sandstone, limestone, and shale. The relative 

 proportion of the ore to the residual material is quite variable, in most 

 of the pits a good average being one part of ore by bulk to five of residue. 

 Locally the yield may reach 75 per cent or more. In very few localities 

 can the ore be handled with profit if it forms less than 15 per cent of the 

 entire mass. The diffused iron oxide is almost universally distributed 

 through the residuary mass over all the limestone areas, portions only 

 of the white clay deposits being free from it. Isolated ore fragments are 

 pretty widety met, only comparatively small areas being entirely free 

 from them ; but areas in which the ore fragments occur in commercial 

 quantities (that is, forming 15 per cent or more of the mass), while 

 numerous, are limited in extent and form a very small proportion of the 

 entire limestone and slate belt. A few of the largest ore banks, such as 



Figure 2. — Section at Pennsylvania Furnace Ore Bank. 

 Showing occurrence of ore in the bedding and joint seams of the limestone. 



the Scotia bank in the Nittany valley, extend over an area of 100 acres 

 or more ; but many are limited to less than half an acre, and some are 

 only a few yards in extent. 



In- the limestone areas the ores occur generally in pockets or solution 

 cavities in the limestone, which are quite irregular in outline and quite 

 variable in depth (see plate 50, figure 2, and also text figure 2). The 

 atmospheric waters have attacked the basset edges of the limestones, 

 which have yielded much more rapidly in some places than in others- 

 The clay, chert, sand, and iron oxide— the insoluble materials — collect 

 in these solution cavities. In a few places the ore may be seen in posi- 

 tion in the seams of the limestone, one of the best illustrations of which 

 is in the Pennsylvania Furnace ore bank, in the Nittany valley (see 

 figure 2), where it occurs in both the bedding and the joint seams, but 

 most abundantly in the latter, which maybe due in part to the fact that 

 they are better exposed to view than the others. The ore deposit is on 



