MECHANICS OF FORMATION OF ARCUATE MOUNTAINS 195 



or dolomite members which are largely devoid of lamination 

 planes and hence to be regarded as not only nearly homogeneous 

 but approximately isotropic as well. Rather generally, however, 

 sediments are characterized by more or less numerous planes of 

 ready separation (lamination) parallel to the original upper and 

 lower surfaces of the formation, and such formations are to be 

 regarded as essentially anisotropic with a minimum of cohesion at 

 right angles to the lamination planes. The movements of particles 

 due to internal stresses are of necessity guided by these surfaces, 

 since the adjacent thin layers permit of the differential sliding 

 (shearing) motions generally described as accommodation between 

 layers. The more perfect the lamination, the more readily does the 

 stratum rise to form an arch. On the contrary, any tendency toward 

 cross-fractures in the strata, such as are generally to be found in 

 shales, lowers the strength of the arch and brings about its failure. 

 The significance of lamination in this connection may be the 

 better appreciated by considering the bending of two rods, one of 

 wood cut parallel to the grain, and the other shaped from some 

 isotropic substance like glass or wax. The rod cut parallel to the 

 grain possesses large cohesion in the direction necessary to resist 

 strong tension upon the convex surface when bent, but the least 

 cohesion in the direction to facilitate the necessary adjustment by 

 shearing between parallel longitudinal layers within the mass. 



Shearing movements within an anticline for the contrasted cases 

 of isotropic and of well-laminated strata. — For the case of laminated 

 strata we may study the internal strains within an antichne by 

 means of a simple experiment. Two exactly similar piles of paper were 

 taken, each some two feet or more in length, two inches in width, 

 and one inch in thickness. Upon the long edges of these piles series 

 of tangent circles were carefully drawn in ink, each with its vertical 

 diameter. The circle was chosen as guide form, both because it is 

 the figure of highest symmetry and favors no one direction more 

 than another, and because conglomerate pebbles, by rudely simulat- 

 ing in some cases this form in section, have sometimes preserved 

 a valuable record of deformations within anticHnal arches. 



Thus prepared, one of the piles was bent into the form of an 

 unsymmetrical anticline (Fig. 30), the other being preserved as it 



