APPALACHIAN FOLDS OF CENTRAL PENNSYLVANIA 249 



Daubree's experiment a triangular prismatic mass has been fractured 

 from the main block, and somewhat lifted. That it did not fold was 

 no doubt due to the absence of adequate gravitative pressure. In the 

 case of the earth-block the gravitative pressures at less than the block- 

 depths exceeded the folding-strength of the strata and the wedge- 

 shaped block deformed instead of simply rising en masse. 



Both theoretical considerations and Daubree's experiments show 

 that angles of fracturing and shearing vary through a rather wide 

 range, especially as the character of the material varies, and the corre- 

 spondence of the Appalachian block to theory is perhaps closer than 

 might be expected. In Fig. 6 the line AB in its deeper portion dips 

 northwest at about 40°, while BC, as it is drawn, dips 54° southeast. 

 In theory the fracture-dips in the upper horizons should normally be 

 lower than 45° while the shear-dip in the lower, less brittle horizons, 

 should be higher than 45°, and this seems to be exemplified in Fig. 6 

 and would no doubt be more strongly shown if the deformation had 

 gone farther. 



As previously stated, the greatest folds in this Pennsylvanian sec- 

 tion lie at the two ends. The computed section in depth shows that 

 these end portions are the thinnest and hence most susceptible to 

 folding. The central portion descends far below the fracture-zone — 

 and perhaps below the zone of typical folding — into the zone of quasi- 

 flowage or plastic deformation, and this no doubt modifies the surface- 

 deformation of this part. 



At the western end the Tyrone fold seems to have been on the eve 

 of passing into a fault when the movement ceased. Farther south 

 faults of considerable throw have actually occurred in like positions at 

 the inland border of the folded tract. In some parts of Tennessee the 

 deformed belt is abruptly terminated on its western side by a sharp 

 thrust-fault beyond which rest undisturbed horizontal beds.' The 

 shell seems here to have been broken by a thrust analogous to that 

 along BC in Fig. 6, and the deformed block thrust outward along 

 the shearing-plane. The undisturbed strata lying just west of the 

 fault indicate that the portion of the shell just outside of the disturbed 

 block, on the west side, has not participated in the deformation. 

 The deep plunge of the shear-plane near the western limit of the 



I Briceville, Tenn. Folio, U.S. Geo!. Surv 



