40 P. IlKPORT OF PROGRESS. E. W. CLAYPOLE. 



through Perry county down the Juniata river to the sea 

 every year. The water basin from which this river sedi- 

 ment comes measures about 10,000,000,000 square yards. 

 Its average loss per year is, therefore, about the ten thou- 

 sandth of a yard. If we take into account the gravel and 

 stones rolled down the river in flood times, and carried down 

 by ice, it will be safe to call it the five thousandth of a 

 yard. 



The whole surface of the Juniata country has, therefore, 

 been lowered say one foot in 1500 years, or 3000 yards in 

 13,500,000 year's ; that is, supposing the climate was always 

 the same, and the Juniata river never did more work than 

 it does now. But as there is every reason for believing that 

 the erosion in the earlier ages was much more violent, and 

 the river far more a torrent, the time required to account 

 for the erosion of the country may reasonably be reduced 

 to ten or even five millions of years, a length of time justi- 

 fied by the vast deposits of the Triassic, Jurassic, Creta- 

 ceous, and Tertiary ages. 



It is this erosion which has revealed so grandly and beau- 

 tifully the internal structure of Perry county, showing the 

 arches and troughs into which the formations have been 

 pressed, and making the present mountains and ridges out 

 of the edges of the harder, and the valleys out of the edges 

 of the softer formations, as will be amply explained in the 

 course of this report. 



The folding of the rocks of Perry county has been so 

 great as to leave scarcel} 7 any of them in their original hori- 

 zontal position. Almost all the strata where they appear 

 at the surface are uptilted at various angles, often very 

 steeply. Some of the folds are so large as to be several 

 iiiil.-^ across : but most of them are so small as to be meas- 

 ured by hundreds of yards or feet ; and there are multi- 

 tudes of still smaller rolls which affect the working of quar- 

 ries and ore mines. But they all belong to one general sys- 

 tem, and are to be explained in the same manner, namely: by 

 a thrust of the whole count ry from southeast northwestward. 



The extent of this thrust can be calculated by taking a 

 cross section of the country (say along the line A B on the 



