RELATIONSHIPS OF ROCK STRUCTURES 819 



strata, and the repetition or elimination of the strata. The general 

 rule can be easily ascertained at any time by a consideration of the 

 simple case shown in Figs. F-G. There it will be readily seen that, 

 when the up-dip end is moved up, elimination results, and when 

 it is moved down repetition occurs, or, to put it the other way, when 

 the down-dip end moves up repetition occurs, but when the down- 

 dip end moves down elimination occurs.* 



Faults as indications of unconformity. 



If in two superposed formations of similar character and sus- 

 ceptibility to faulting the lower is more complexly faulted than the 

 upper, the indications are that the lower was faulted before the 

 upper was deposited upon it, and that then the two formations are 

 unconformably related. 



Relation of folds, faults, cleavage, Ussility and joints. 



Van Hise has emphasized the close relationships existing be- 

 tween these structures, which may in general be considered as dif- 

 ferent manifestations of the same forces — thrust and gravity — act- 

 ing upon heterogeneous rocks under varying conditions. When rocks 

 are under less weight than their ultimate strength, while being rap- 

 idly deformed, they will break, with the formation of crevices, of 

 joints, faults, brecciations or fissility, as a result of extensive frac- 

 turing. Such rocks are then regarded as being in the zone of frac- 

 ture. When rocks are buried to such a depth that the weight of the 

 superincumbent strata exceeds their ultimate strength, they will 

 flow as plastic material under deforming strains and folding with- 

 out fracture results. The depth at which this takes place marks 

 the position of the zone of plasticity and Uozvage. The depth at 

 w^hich flowage occurs varies with the character of the rock. For 

 soft shales, \'an Hise estimates that probably 500 meters or less of 

 overlying strata will prevent the formation of crevices and fractures 

 to any considerable extent, while for the strongest rocks a depth 

 of perhaps 10,000 meters is required to reach this condition. Cleav- 

 age normally belongs in this zone. Between these two is the zone 

 of combined fracture and plasticity. In this zone all the structures 

 occur together in complex relationship. Folds may pass into faults 

 and faults into folds. Fissility and cleavage occur side by side. 



* This may be condensed into the slogan — down, down, out. 



