MODE OP ACCUMULATION 497 



the limestone and in the slate, as already shown (see figures 2 and 4). 

 It is broken up and mixed with the other residua by the leaching away 

 of the surrounding and underlying limestone and by the whole mass 

 sinking down to a lower level. An important horizon for the deposition 

 as well as the accumulation of the ores is at the contact of the limestone 

 with an underlying insoluble layer, such as slate or sandstone. 



Some of the ores are oxidized concretionary masses which have formed 

 in the rock before it was disintegrated (see figure 1). I did not see any 

 nodular ore masses in position in the limestones of the area under dis- 

 cussion ; but the nodular ore fragments are frequent in the residual 

 material, and they evidently were formed either in the limestones or in 

 the intercalary clay layers. 



The accumulation of the clay-ore masses is aided in part by the in- 

 equalities in the dissolution of the limestone, and in part by the occur- 

 rence of insoluble beds of clay or sand intercalated in the limestones. 

 Thus there is a tendency, in the first place, as previously stated, for the 

 ores to collect in caverns, and as their roofs break through or are dis- 

 solved by the waters, there is a further tendency for the loose materials 

 immediately surrounding the cavity to work into it by gravitation, and 

 the ores, being heavier than the clay, tend to work deeper, toward the 

 bottom of the opening. 



The interstratified clay and sand beds aid in the concentration of the 

 ores by forming an insoluble layer on which the ores may collect from 

 the overlying limestone. Thus a bed of clay, as shown in figure 3, may 

 serve as a final repository for all the ores of the overlying limestone bed, 

 whether it be 200 or 2,000 feet thick. There will also be a lateral move- 

 ment down the inclined bed of clay, slate, or sandstone. Thus, in figure 

 3, for example, all the ore from the triangular space A C B may be col- 

 lected in a comparatively small area at C. Should there be a cross- 

 folding in the other direction, at right angles to the main fold, so as to 

 form a synclinal trough down the slope B O, the concentration will be 

 all the greater. We have here an explanation of the rich ore deposits 

 on the clay beds in the Great Valley area and on the sand deposits in 

 Nittany valley. It is possible for the concentration to be going on at 

 two or more levels at the same time. Thus in the figure cited the con- 

 centration may have begun at C, while it is still going on at A ; but 

 eventually the upper deposit will be brought to the lower one by the 

 leaching out of the intervening limestone. If the clay seam at A is com- 

 paratively thin, it may lose its identity in the mixture of other materials 

 before reaching the lower level. 



If this explanation be the true one, the newest ore, as a rule, will be 

 at the bottom, and the ore deposits will increase in size by additions 



