266 University of California Publications in Geology [Vol.13 



Joints. — The vein fissures are major joints of a type heretofore 

 unrecognized, not being cooling cracks but being clue to mechanical 

 stress. They characteristically span the minor folds which pitch down 

 the limbs of the major folds, more generally the minor synclines, or 

 they lie along the axes of the minor anticlines. They also frequently 

 lie along the dips of the major folds. They vary in length from 

 perhaps 100 to 1000 feet, and in height from 50 to 500 feet. In their 

 characteristic position, spanning minor synclines normal to their axes, 

 they occur generally in groups of from two to a dozen spaced at 

 intervals of not less than 30 nor more than 100 feet. Groups of this 

 kind generally occupy the part of a minor syncline which has suffered 

 the most acute compression. Their occurrence in these peculiar situa- 

 tions, parallel to the folding stress which produced them, has led me 

 to name them split joints. 



Major joints of this type have probably escaped the notice of other 

 geologists, because by the time they usually become exposed to view 

 on the surface by its degradation, the rock strains have been relieved 

 by many lesser joints which are much more closely spaced, and thus 

 obscure the manifestation of this older, less numerous, and more sig- 

 nificant type. The recognition of them at Cobalt was unavoidable 

 from the fact that they are virtually the only joints mineralized, since 

 they were the only ones open at the time and place of mineralization. 

 The fact of their containing veins often of pure silver was the extra- 

 ordinary circumstance that led to their being scrupulously followed 

 by mine workings for hundreds of feet along their strikes and dips, 

 and thus brought to light. 



Other types of mineralized joints are of less importance, but 

 require mention; they are: (1) fairly strong joints of either no dis- 

 placement or very slight displacement parallel to and in the walls 

 of inclined faults, (2) strong vertical joints branching from inclined 

 faults, but parallel in strike, (3) lesser joints, vertical, branching from 

 inclined faults, and parallel to their dip, (4) strong joints branching 

 from strike-slip faults, nearly parallel in dip, but diverging in strike. 



All other joints are uncemented and have obviously originated 

 after the period of mineralization. They are of two general classes, 

 namely, those which are related to the cemented joints, and those 

 unrelated. Each typical cemented joint is paralleled in its walls by 

 one or more of the first class of uncemented joints ; and each fault 

 wall contains many and various joints of the same first class. Of the 

 second class there is one type which bounds rhombohedral blocks situ- 



