

LETTER OF TRANSMITTAL. t . Vll 



law first discovered by Dr. Whelpley in 1838, that the pli- 

 cations of a great system of rocks are not regular from top 

 to bottom ; that the set visible at the surface does not cor- 

 respond exactly to those concealed beneath the surface ; 

 and that a few great folds in the massive sand-rocks are 

 represented by a much larger number in the overlying or 

 underlying soft formations. 



Thus, the two great basins and the great intermediate 

 arch in the upper formations at the east end of Perry 

 county are represented by many smaller arches and basins 

 in the lower formations at its western end ; and we may be- 

 lieve that if we could get data for constructing a true cross- 

 section on a plane 10,000 or 15,000 feet beneath the Susque- 

 hanna river it Avould exhibit a similar series of small 

 arches and basins. 



I have also endeavored to show, on Plate IV and on 

 other plates, how faults at the surface must necessarily 

 terminate downward in unbroken folds whenever the dip 

 is stronger on one side of the fault than on the other, no 

 matter at what angle the fault-plane itself may stand ; that 

 the upthrow must necessarily take place on the side of the 

 steeper dip ; and that it must increase upward constantly. 

 In horizontal rocks, in implicated regions, faults may go to 

 great depths ; but in folded regions they must stop at some 

 definite distance underground easily calculated by merely 

 drawing a geometrically true cross-section. 



The decided north-westward leaning of the Appalachian 

 folds is shown in these Perry county sections ; and it is 

 well to call attention again to the smaller and more numer- 

 ous folds in the lower formations in connection with this 

 north-westward leaning, on the supposition of its being 

 caused by a thrust from the south-east. No satisfactory 

 explanation of the origin of such a thrust has offered itself. 

 The fact itself seems to be undoubted ; but whether the 

 movement was one of the underlying Azoic foundation 

 rocks, or whether it was confined to the superficial paleo- 

 zoic formations sliding down a rigid inclined plane pro- 

 duced by the vertical elevation of the Azoic zones, is the 

 first question. In the latter case it is easy to see how the 



