862 STUDIES FOR STUDENTS 



rial from the limbs to the arches, the limbs becoming thinner and 

 the arches thicker by this process. Foliation planes, or planes 

 of easy separation, are also produced, which in moderately com- 

 pressed folds make small angles with the axial plane of the 

 folds, 1 but in closely compressed folds they are approximately 

 parallel to that plane. 



MECHANICS OF DEFORMATION BY RUPTURE. 



The problem of deformation by lateral compression of a 

 crustal block has been treated from a mathematical standpoint 

 by Becker, 2 his discussion assuming the section of crust to be an 

 elastic solid. 



This block may be regarded, then, as subjected to stresses 

 from which it acquires the properties of an anisotropic medium 

 in which the greatest and the least axes of the strain ellipsoid 

 lie in the horizontal plane, owing to the fact that gravity opposes 

 an elongation of the particles in the vertical direction. The 

 effect of such stress is to produce a system of vertical planes of 

 dislocation which in a mass of infinite resistance (perfect elasti- 

 city) would make 45 with the direction of pressure, but other- 

 wise (if more or less plastic) the angle would be less. With 

 pressures rapidly applied it is believed that rocks behave like 

 highly elastic bodies. The best evidences that such is the case 

 seem to be furnished by the almost universal observation that the 

 most prominent joint planes are nearly vertical (except in subse- 

 quently tilted rocks) and so generally nearly perpendicular to one 

 another; 3 and by the experiments of Crosby, 4 who found that a 

 system strained to a point far below that of rupture (near which 

 plasticity becomes for the first time an important factor) is 

 relieved from its stress by dislocation through shock communi- 

 cated to the system (for example, by an earthquake). In the 



1 See this Journal, Vol. X, p. 787. 



2 Geo. F. Becker, "Finite Homogeneous Strain, Flow, and Rupture of Rocks," 

 Bull. Geol. Soc. Am., Vol. IV (1893), p. 50; see also Van Hise, loc. cit., p. 672. 



3 A. Daubree, Etudes synthetiques de geologie experwientale, Part I (Paris, 1879), 

 pp. 300, 301 ; see also Becker, loc. cit., p. 73. 



4 W. O. Crosby, "The Origin of Parallel and Intersecting Joints," Am. Geol., 

 Vol. XII (1893), pp. 368-75- 



