DEFORM A TION OF RO CKS 6 3 



specimen may show the original bedded structure and secondary 

 structures in two or more different directions. In fact, as has 

 been shown, a single simple orogenic movement may produce 

 both cross and parallel secondary structures, and the cross fissility 

 may be in two directions. 



While it is rare to find more than two or three structures in 

 a rock, theoretically there is no limit to the numbers which might 

 be produced ; but practically, as has been seen, new movements 

 usually emphasize old structures, or else produce new structures, 

 which tend to obliterate the old structures. Hozvever, in a thor- 

 oughly crystalline rock, if there be two or three structures, it is ?iot safe 

 to assume that the oldest and most intensely plicated ofie is bedding. 

 This has been done frequently in the case of the crystalline 

 schists and gneisses by those who would not regard cleavage or 

 foliation, if but a single structure existed, as evidence of bedding. 

 The older the structure the greater is the probability that it is 

 really bedding, but the fact that it is the earliest structure which 

 now exists in the rock cannot be regarded as conclusive, for it 

 may have been produced by an early orogenic movement which 

 simultaneously obliterated bedding. 



After a secondary structure has developed in a forma- 

 tion it ma}' be folded into anticlines and synclines. In order to 

 be thus folded it is usually necessary that the secondary struc- 

 tures be not steeply inclined. As indicated on a previous page, 

 where such a structure develops in a horizontal position it may 

 correspond with bedding, but also it had been seen that cleavage 

 or fissility may form with slightly inclined planes of movement 

 which cut diagonally across the bedding. Such a cleavage or 

 fissility may be emphasized by secondary impregnations and 

 injections, in which case it simulates bedding to a remarkable 

 degree. In eithei of the above cases a careless observer would 

 be almost certain to regard the structures as bedding. 



The number and severity of orogenic movements may in 

 many places have been so great along old ranges that it is not 

 strange that it is impossible to differentiate or separate the 

 various formations upon a structural basis. The beds have 



