GEOLOGICAL HISTORY OF THE COUNTY. F\ 35 



Five miles west of Green Park, a cross-section through 

 Centre Village shows that the southern trough has passed 

 out of Perry county into Cumberland, as also the axis and 

 southern half of the middle arch. See Fig. 4. 



Six miles west of Centre Village, a section of the north- 

 ern trough through Beavertown restores it to its original 

 simplicity. See Fig. 5. 



Ten miles west of Beavertown, the sorthern half of the 

 great anticlinal is magnificently plicated. See Fig. 6. 



However simple then may be the general structure of 

 Perry county, its complications in detail are numerous and 

 very important, causing many local repetitions of the out- 

 crop of each rock, and giving a curious and beautiful zigzag 

 pattern to the colored geological map. 



Faults. 



While the two great troughs have bent to all appearance 

 without breaking, the intermediate great arch suffered not 

 only numerous plications, but at least three important frac- 

 tures, one of which is sixteen miles long. But, as the 

 character of these up-throio faults can not be understood 

 without a knowledge of the rock-formations affected by 

 them, their description is postponed to a following chapter, 

 Chapter IV. 



Geological history of the district. 



At the earliest date to which Geology can point back with 

 tolerable certainty in the history of what is now Perry 

 county, the interior of the North American Continent was 

 an ocean of unknown extent into which were borne the sand 

 and mud of neighboring lands, swept down by the rivers of 

 that distant age to make the beds of rock which to-day 

 compose the solid land of the United States. 



The history of this process is written in the rocks, and a 

 brief sketch of the history is essential to the intelligent 

 perusal of this report. 



In the following table are given the names of the differ- 

 ent formations which, lying one upon another, are vn\w 

 posed of the sediment washed at different dates into this 



