VEINS. 409 



pass over into folds, as several geologists have pointed out; and even in 

 granite areas dikes of eruptive rock may soniefimes be ol)served which 

 diminish gradnall>' in width to a tine edge and disappear. There is every 

 probability that fissures also die out in depth in a similar manner, though 

 where considerable faults have occurred the dej)tli of a fissure must be very 

 great. Many fissures certainly penetrate from the surface of the earth to 

 the foci of volcanic activity, a depth probably at least equal to that at 

 which earthquake shocks originate, or several miles from the surface. 



Simple veins, parallel veins, and linked veins. WllCU the tOrm (isSUrC Vciu is UScd 



without any qualilication, it brings to mind a very simple and common 

 form of deposit: a fissure with well defined walls usually nearly straight 

 or curving gradually and including vein matter which is connnonly com- 

 posed of ore, gangue, and fragmentary mas.ses of country rock. A vertical 

 section of such a vein is shown below (see Fig. 20 n). Tt appears to me very 

 desirable not only to call a deposit of this kind a simple fissure vein, but to 

 limit the application of this term to deposits of this kind. It is not difH- 

 cidt to find natural designations for allied but less regular depo.sits. 



Where the formation of a fissure is accompanied by a strong compress- 

 ive stress groups of parallel fissures form, often passing over into a com- 

 mon fold at each end. Tlie dislocation is then distributed ovei- a luimber 

 of parallel surfaces instead of a single surface, and this distribution takes 

 place according to a definite law, which 1 have examined. on other occa- 

 sions.^ In some cases such fissures form with great i-egularity and are dis- 

 tinct from one another as far as they can be traced. If ore-beai'ing solu- 

 tions enter such ground, they deposit distinct, parallel veins. Such deposits 

 are naturally described as groups of parallel veins. 



In many cases a tendency to the formation of groups of parallel fis- 

 sures is obstructed, perhaps by irregularities in the tenacity of the rock or 

 by the action of complex forces. In such instances approximately parallel 

 fissures form, Avhich die out in the direction of their strike, being replaced 

 by others to one side or the other. More or less diagonal stringers must 

 then exist, connecting the principal crevices. Sometimes fissures of this 



■ Geology of the Coiustock Lode Chapter IV; Imii.act, friction, and faulting: Am. Jour. Sci., 3d 

 series, vol. 30, ISS.'i. 



