MODE OF OCCURRENCE 



487 



This may be caused by a segregation of the ores in underlying lime- 

 stones, and the subsequent leaching out of the intervening rock permits 

 the upper cla\' to rest on the ore, as shown in figure 3. Should the 

 second accumulation. rest on clay the final result is an ore body inclosed 

 in the clay. In some instances the cause of the ore being beneath the 

 clay is an overturn or an overthrust of the strata. Structural relations 

 support this view at several localities on Mountain creek, in Cumber- 

 land county. Sometimes it is simply irregularity in weathering. 



While usually the ore deposits rest on the white clay, occasionally 

 they occur in it, and in one place, Hensingerville, the ore is found im- 

 pregnating the hydromica slates from which the clays are derived, thus 

 showing how the ore may get into the clay. In the joint and lamina- 

 tion seams of the slates the ore occurs in thin flakes, which resemble 

 those mentioned in the limestone at Pennsylvania Furnace, except that 



Figure 4.— Cross-section of Ore Bank at Hensingerville, Pennsylvania. 

 Illustrating accumulations of ore in the hydromica slates. 



they are smaller and more numerous, just as the seams of the slate are 

 more numerous and closer together. The exposure is on the south wall 

 of an old ore pit (see figure 4), and the interlamination of the ore extends 

 at least to the depth of the pit, 60 feet below the surface, and presum- 

 ably deeper, as the pit opening, extending more than 100 feet north, was 

 formed by the removal of the clay for the ore contained therein, and 

 which must in part, at least, have come from slates underlying those 

 exposed. When the workmen making the excavation came in contact 

 with the but partially disintegrated slates on the south face, the mining- 

 was no longer profitable and operations ceased. A shaft 50 feet back 

 from the quarry face shows a similar occurrence of the ore flakes in the 

 seams of the slate to the bottom of the shaft, 15 feet below the surface. 

 In this mine the bombshell or nodular ore was quite abundant, forming 

 concentric shells in the slate and the resultant clay (see figure 1). 



