STRUCTURAL RESULTS OF FAULTING. 



175 



it may be "only as small alterations of form in the body acted upon " 

 Rocks are by no means absolutely rigid or absolutely inelastic, and under 

 the conditions supi)Osed a strain must be produced in the hanging- wall. 

 Sedimentary strata, and especially the coal measures, furnish innumerable 

 known examples of this action, indicated by tlu! permanent flexure of the 

 ends of the strata as indicated in Fig. 9. This is of course a familiar fact 

 which has from time imme- 

 morial furnished miners with 

 a practical rule for recovering 

 the seam beyond a fault. 



When a fault takes place 

 in the comparatively rigid 

 massive rocks a similar strain 

 must also be produced. Its 

 effect will depend ujjou its in- 

 tensity and on the elastic pro- 

 perties of the rock. These 

 latter are so little known that 



it is scarcely worth while to !■''«• 9.-Faultaccompauied by a stia,.,. 



investigate the conditions mathematically, but it is certain that if the strain 

 surj^asses a limit defined by the cohesion of the rock, a sheet of the latter 

 will be sheared off from the main mass. If the compression attending a 

 fault in a massive rock is very great, and if the rock is very rigid, this action 

 may be repeated indefinitely, and either oi- both walls may be divided into 

 sheets of nearly equal thickness and divided by partings nearly parallel to 

 the original fissure. On the other hand, if the stress does not reach the ulti- 

 mate cohesive resistance of the rock, the energy must be expended in heat 

 and a strain which will be permanent or not as the rock is elastic or in- 

 elastic. 



Evidence furnished by observation. — lu coal mlues there is abuudant evidence of 

 permanent strains produced by faulting. In massive rocks a division into 

 sheets sometimes accompanies faulting, but it might be asserted that the two 

 phenomena were unconnected. A very unobtrusive structural action serves, 

 however, to establish a relation. In hilly regions where the soil is deep, small 



