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University of California Publications in Geology [Vol. 13 



When rocks are folded at a considerable depth below the surface, 

 or under certain other conditions of uniform horizontal compression 

 throughout a considerable vertical column, so that parallel folds are 

 not possible, and only similar folds 3 can be produced, shearing must 

 develop on their flanks to accommodate the shifting o'f material from 

 the limbs to the axes of the folds. As pointed out in a previous 

 paragraph the axes of folding are also neutral axes with reference to 

 shearing, and the regions of maximum shear are midway between 

 them. The direction of shearing is that of dip-slip reverse faulting 

 parallel to the warped surface. The surfaces of shearing in a thick 

 homogeneous formation are distributed throughout its thickness, and 

 are probably of small individual area, the displacement on each one 

 being small; but in a flat bedded formation they probably coincide 

 with the beds. In either case, here and there all local strains will be 

 concentrated upon a single surface, which will, in consequence, have a 

 large displacement and a large area. In such a case the recognition 

 of a fault is an arbitrary matter depending upon the quantity of dis- 

 placement adopted as critical for the definition of a fault; and all 

 surfaces of shearing of less magnitude must be classed as joints. In 

 the absence of any better means of designation, I shall refer to these 

 as shear joints. 



During deformation strains must tend to be concentrated in regions 

 of weakness, relief of one strain precipitating others until all local 

 stresses are relieved, down to the limiting strength of the material. 

 In this manner, the contacts between formations, being lines of initial 

 weakness, must become the chief loci of relief of strain ; and in their 

 vicinity faults and joints must be most abundant ; this is notably 

 true at Cobalt. This rule, however, is variously limited, and particu- 

 larly applicable to contacts which were flat or nearly so before folding 

 began, the shearing stresses due to folding being concentrated largely 

 upon them, the very steep contacts being virtually immune. As would 

 be expected the shearing and fracturing on the limbs of folds is pro- 

 portional to the amount of folding. 



The Cobalt Lake fault is the only known local representative of 

 either of the major systems, and is the chief reverse fault that is not 

 related to the folding shear strains, having an oblique shift of 500 feet, 

 at an angle of 25 degrees with the dip, toward the west. All other 

 reverse faults are related to the folds, and dip with their dips at the 

 same or steeper angles, usually striking approximately with their 



3 C. K. Leith, Structural geology, p. 107. 



