254 F. H. Lahee — Metamorjphism and Geological Structure. 



reaching these where the folds are most acutely pinched. In 

 similar folding maximum shearing is always in the limbs.* 



If strata, deformed after the parallel pattern, have a pitch, 

 carinate anticlines become flat and flat synclines become cari- 

 nate, when traced in the direction of pitch ; but in similar 

 folding, since dips are always steeper on the limbs than on the 

 axes, no amount of pitch can alter these relations. Parallel 

 folding, representing less readjustment of beds than similar 

 folding, is more common and is generally on a larger scale 

 than the latter. It must be remembered that these state- 

 ments apply to mathematical ideals only, and that, under nat- 

 ural conditions, there is considerable variation. The two 

 types are not always sharply distinguished ; yet there is suffi- 

 cient approximation to the ideal to make the classification 

 valuable. 



Given a force in operation, a more rigid body will oppose 

 deformation by this force more successfully than a less rigid 

 body. If adjacent rocks of different degrees of rigidity are 

 under lateral compression, whether the forces be regarded as 

 acting parallel or perpendicular to the contact surfaces 

 between the rock masses, there is a tendency- for transmission 

 of these forces by the stronger body.f The first condition — 

 of force parallel to contact, i.e., about parallel to the beds — is 

 that for the development of competent structure'^, the second 

 condition — of force about perpendicular to contact — is illus- 

 trated by the relations between hard crystalline border-rocks 

 and less resistant basin sediments, after deformation of the 

 original land surface has progressed far enough. In nature 

 the differences of rigidity are practically never so great that 

 one rock merely transmits the force while the other accom- 

 plishes all the accommodation. Both usually suffer, but one 

 less than the other. 



The more rigid a rock mass under compressive strain, the 

 farther from the point of application of the force will the 

 effects of that force appear. For this reason, unless a stratum 

 has competency sufficient to enable it to span the breadth of 

 the deformed belt, the folds are apt to be closer and more 

 numerous near the point of application and to die out away 

 from it ; § and the less the rigidity of such a stratum, the 

 more rapidly will the folds subside. 



* Van Hise, C. R., "Principles", etc., p. 598. 



f Harker, A., On Slaty Cleavage . . . , Rept. Brit. Assoc. Adv. Sci., 

 1885, p. 848. Heim, A.: Op. cit., p. 40. 



Van Hise, C. R., Deformation of Rocks, Jour. Geol., iv, pp. 204, 472, 

 1896. 



^Willis, B., The Mechanics of Appalachian Structure, IT. S. G. S., Ann. 

 Rept. XIII, Pt. II, 1891-1892, p. 247. 



§Shaler, N. S., etc., op. cit., p. 16. 



