276 University of California Publications in Geology [Vol. 13 



On several of the lesser faults of the district evidence in the way 

 of superimposed grooves on slickensided surfaces indicates movement 

 in different directions at different times on a single fault. In view 

 of such other evidence as reversed sequential relations in minor faults 

 and joints, it seems clear that stresses on the complementary structural 

 axes must have been active alternately in one direction and then in 

 the other. From the orientation of the structures as described, it 

 appears that the principal structural stresses must have been in a 

 general sense simultaneously active throughout the period of deforma- 

 tion which immediately followed the injection of the diabase. This 

 being true, there must have been times when there was simultaneous 

 stress from two oblique directions producing torsion effects ; and this 

 is borne out by numerous examples throughout the district, of groups 

 of small gapping fractures arranged in echelon. Whether these 

 stresses sprang from the same source as those which produced the 

 major complementary structural lines, or resulted from movement 

 upon them, or whether they were directly due to the shrinkage of the 

 diabase and its environs due to the loss of heat, is difficult to decide ; 

 but whatever the source, it may be presumed to have been not spas- 

 modic and variable, but continuous, the apparent alternations being 

 due to the fact that the media under strain were of various strengths. 

 The network of stresses over the region would thus be finding relief 

 in a given direction at various points at the same time, and at inter- 

 mediate points simultaneously in the complementary direction ; here 

 and there torsion would arise from the relief of strain in both direc- 

 tions at the same time and place. Strain resultants might also be 

 expected ; and fractures thus oriented are not uncommon. 



Minor structures. — The most significant of the minor structures 

 are the split joints, which occur most characteristically in groups 

 spanning the minor synclines, or extending along the axes of minor 

 anticlines, or which occur singly and less frequently striking with the 

 dips of the limbs of major folds. In detail, a split joint is typically 

 a single strong major joint ; but frequently it consists of a pair or 

 triplet of fractures, or a chain of branching and reticulate fractures 

 having a zonal width of between five and fifteen feet. 



The split joints in a major syncline have difficulty in completely 

 spanning the structure, and are generally related to the limbs rather 

 than to the structure as a whole ; but a minor syncline is usually com- 

 pletely spanned by them, and on its limbs they will intersect not only 

 the shear joints developed on the sides of the major syncline down 



