104 SOUTHERN CALIFORNIA GEOLOGY 



as their tilting, bending, dislocation, faulting sequence, 

 etc. Some oil geologists use the term "structure" 

 merely for earth folds in which oil is found, but it 

 is herein used in the broader sense. 



The faulted types of structure are most evident in 

 some physiographic features and the folded types in 

 others; while in still other instances both types 

 are associated. Folds sometimes pass into faults in 

 their longitudinal continuations; in this vicinity they 

 are unusually paralleled on one side by them. Likewise 

 folds which are closely paralleled by faults sometimes 

 lose their identity as folds and appear as fault blocks. 



In Southern California the structure consists to some 

 extent of arches (folds) of strata, which may bend 

 either upward or downward (anticlines and synclines) 

 but more conspicuously of long fractures or rifts,^ 

 called "faults", along the opposite sides of which the 

 rock material has been displaced. 



REGIONAL TYPES OF STRUCTURE 

 Variations in the types of structure and their details 

 are visible in different regions of the world. In some 

 portions the folded type of structure predominates, 

 as is seen in the mountain regions of the Jura and 

 the Alps, in the Appalachians and in the Rocky Moun- 

 tains. In other regions, particularly the one in which 

 we live, faulting appears to be a more dominant type 

 of structure than folding. 



Both sides of the great Pacific oceanic basin are 

 bordered by lands where the fault-rift type of struc- 

 ture prevails, and which record subsidences of the floor 



^The term rift is herein used in its ordinary dictionary meaning, to-vvit, 

 a rip, a crack, a tear, or a fissure. Such features are also usually fault 

 lines, where the materials of one side of the rift have moved relative to those 

 of the other. 



