APPALACHIAN FOLDS OF CENTRAL PENNSYLVANIA 247 



upper five or six miles of strata, the strains appear to have diverged 

 into an increasingly thick shell. In other words, the shear plane 

 plunged downward. After reaching the maximum depth beneath 

 Section 2, the limit of the deformed mass was deflected rapidly toward 

 the surface. The deformed block thus assumed, in a general way, 

 the form of a triangular prism. 



That the analysis of field data should develop a deformed block 

 of this shape and attitude was wholly unexpected but behavior of 

 this sort is, in reality, entirely in accord with what the principles of 

 mechanics imply for such bodies under lateral stress. In solid 

 bodies under direct pressure, fracturing and shearing usually take 

 place along the planes of greatest tangential stress. Becker has 

 developed the application of this principle mathematically in the 

 case of strained rocks. "A direct, uniformly distributed pressure of 

 sufficient intensity, applied to an elastic brittle mass presenting great 

 resistance to deformation, would induce fracture. The ruptures 

 would take place along those lines subject to the greatest tangential 

 strain, since these are the directions in which material would first 

 be strained beyond endurance. These lines would stand at 45° to 

 the line of force if the mass presented infinite resistance to deforma- 

 tion."' Hoskins in his analysis of strain and stress applied to the 

 flow and fracture of rocks has also discussed this principle: "Simple 

 sliding at any instance takes place along two sets of planes at right 

 angles to each other and inclined 45° to the directions of elongation 

 and shortening at that instant."- Leith's doctrine of fracture-cleavage 

 is dependent upon the same principle and he agrees with Becker on 

 the fundamental principle involved: "If fractures occur in irrotational 

 strains, these follow intersecting planes approximately 45° to the great- 

 est pressure — planes of greatest tangential stress."^ The exact angle, 

 however, varies somewhat with the nature of the substance and with 

 the stress-conditions. In the Appalachians the greatest mountain- 

 building pressure acted essentially horizontally. The planes of great- 

 est tangential stress should, therefore, dipat angles somewhere in the 



1 G. F. Becker, "Finite Homogeneous Strain, Flow and Rupture of Rocks," 

 Bull. Geol. Soc. Amer., Vol. IV (1893), p. 50. 



2 L. M. Hoskins, " Flow and Fracture of Rocks as Related to Structure," Six- 

 teenth Ann. Rept. U.S. Geol. Surv. (1894-95), Pt. I, p. 865. 



3 C. K. Leith, "Rock Cleavage," Bull. 2JQ, U.S. Geol. Surv. (1905), p. 121. 



