TEREANES. 107 



Rocks vaiy greatly in fragility, and break very differently under the same 

 circumstances. Some, as the shales and argillaceous sandstones, yield to 

 pressure by bending or by becoming compressed or stretched ; or, like a 

 friable sandstone, become adapted to the pressure as might a bag of corn, 

 by a readjustment of the grains. But the more solid sandstones, having less 

 mobile elements, become divided into blocks, unless the pressure at work is 

 of the extremest slowness ; and compact limestone, the most brittle of rocks 

 when pure, breaks into smaller blocks, and sometimes into multitudes of 

 them. The fractures due to stretching or tension are often large in the 

 summit portion of an anticline, and especially when the beds consist of the 

 harder or more brittle rocks. If a stratum of limestone is made up of pure 

 and impure (argillaceous) layers, the former may be broken into columns when 

 the latter are sparingly broken. Only a slight torsion from unequal pressure 

 or support is needed for these results. The scenery of the Rocky Mountain 

 region, and especially of the Colorado Caiion, illustrates finely these various 

 differences in fragility. It is dependent upon the columnar fronts of many 

 of the harder alternating layers for much of its architectural effect. 



It has been stated that the flexures in strata are those of a warped sheet. 

 But while the coat-sleeve loses its flexures on straightening it, strata could 

 not be restored to their original condition, because of the great stretching 

 and slipping on one another of the beds in one part, and of the compression 

 in others. Proofs of the stretching and compression are afforded by the 

 deformation of fossils, as illustrated in the chapter on mountain-making. 

 (See page 370.) The smallest of fractures that have geological importance 

 are those of the constituent grains or crystals of a crystalline rock, which 

 are generally so minute as to be detected only by microscopic investigation. 

 They sometimes indicate a flowing of the material, lava-like, before it had 

 cooled, or contraction during cooling, or some progressing change of form 

 through pressure. 



Faults are displacements along fractures. When a coal-bed is not con- 

 tinuous across a plane of fracture, but has its continuation at some higher or 

 lower level, a fault exists ; and such faults often occasion much trouble to 

 miners. There may be a few inches or less of disjylacement, or a few feet ; 

 but the larger faults of mountain-making regions are sometimes 10,000 to 

 20,000 feet. 



In Pigs. 106, 107, ft is the course of a fracture, and a to 6 the amount of 

 displacement. In Fig. 106 the part to the right has slipped down against the 

 opposite wall, or there is a downtJiroiv ; and this downthrow is in the direc- 

 tion of the dip (or the hade, in miners' language) of the fracture-plane. In 

 Fig. 107, the reverse is the case ; there is an upthrust along this plane. One 

 is an overthrow or downthrust fault, the other an upthrust fault. The angle 

 of dip in the fault-plane is here near 60° ; but it may be from 0° to 90°. 



In Fig. 108 a block of the formation has slipped down between two frac- 

 ture planes ; moreover, the resistance or friction has produced a bending of 

 the layers on one side. Reverse the figure, and another condition in faulted 



