GEOLOGY — CHAMBERLIN. 1 77 



folding of the Appalachian tract of Pennsylvania west of Harrisburg, 

 according to Lesley's estimate.* The application of this method to 

 other mountain ranges for which approximate estimates of the crustal 

 shortening and the height of the folded tract are at command gives 

 results of a similar order of magnitude. Recognizing the imperfec- 

 tions of such estimates, it may be assumed for the purposes of further 

 consideration that the superficial shell involved in folding is of the 

 order of 3 to 5 miles in thickness rather than of dimensions of a 

 markedly higher order, such as the supposed thickness of the crust of 

 the earth overlying a molten interior or a mobile substratum. The 

 considerations that lead to this conclusion are applicable to an earth 

 of any internal constitution unless it be supposed that the folding 

 involves inward flexure of such an order as to produce a widespread 

 displacement instead of contributing to a definite external protrusion. 



It is obvious from this and from the further considerations that 

 will presently be offered that the determination of the amount of 

 crustal shortening involved in mountain folding and faulting, together 

 with the resulting external protrusion, are elements of importance, 

 and that more careful determinations are eminently desirable. This 

 has led to a consideration of methods for such determinations, and 

 some devices not known to be in use have been developed and put to 

 preliminary tests. It is thought desirable, however, to carry these 

 further before committing them to print. 



The deduction that the shell involved in mountain folding is so 

 limited in depth seems to carry the necessary corollary of a shear 

 zone between it and the great body of the earth. This zone must 

 lie within the horizon of solid rock, for under no tenable hypothesis 

 can a molten stratum be assigned to so shallow a depth. This shear 

 zone appears to lie at or above the junction between the zone of 

 fracture and the zone of flowage deduced by Van Hise and Hoskins 

 from the relations of vertical pressure to the crushing strength of 

 rock, which is placed at a depth of about 10,000 meters. If we seek 

 for a cause for shearing at this horizon rather than a higher or lower 

 one, it is probably to be found in the relationship between the weight 

 of the overlying rock and the shear resistance of the rock at the given 

 horizon, very much as the limit between fracture and flowage defor- 

 mation is found in the ratio between the weight of the overlying rock 

 and the crushing strength of the rock. The shearing strength of 

 rocks representative of the horizons involved is scarcely known at 

 all experimentally, but it may theoretically be assumed with confi- 

 dence to be somewhat less than the crushing strength, and hence the 



*2d Geol. Sur. Penn., Pt. X, p.-xxi, 1885; also Summary, Final Rep., p. 206, 1903. 



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