CHARACTER AND FILLING OP THE VEINS. 229 



to which these fissures are duo, wliile bearing a genonil similarity to 

 those manifested in the cleavaue, ofton dilTcred from them in direction 

 to a sensible extent. 



Different rocks intiuence the character of the fissures to some extent. 

 In massive rocks they are apt to be straight, clear cut and well defined ; 

 in slates and serpentines there is often a tendency to splinter into a net- 

 work of cracks and fissures, extremely small, but often ver}' rich. In 

 such cases the whole mass, country-rock and vein, may be extracted and 

 milled. Linked veins are common and chambered veins* sometimes 

 occur. 



Ver}' long and continuous veins are not common, and in this respect 

 tlie mother-lode is rather an exception. Only rarely can a (juartz-vein 

 be traced more than a few miles, and many important veins vvo[) out 

 only for a short distance.f 



The Filling of the Veins. 



The typical gold-quartz veins cannot be considered as anything but 

 fissures and fractures filled with quartz, accompanied by small amounts 

 of native gold and metallic sulphides. Replacement pro[)er of the min- 

 erals of the country-rock along the fissure by quartz I have never been 

 able to observe, and cases supposed to be of such nature have always 

 proved to be due to the shattering of the country-rock and the filling of 

 it by silica along narrow cracks. The clean quartz usually forming the 

 vein I cannot account for in any other way than by filling of cavities, as 

 it does not seem possible that a rei)lacement of the ferro-magnesian sili- 

 cates and otlier minerals could occur without leaving chloritic stains or 

 other signs in the resulting mass, in all quartz-veins of this type it 

 seems unavoidable to admit the existence of open spaces along the yein, 

 supported at frequent intervals by the contact of the two walls or by rock 

 fragments. Even the heaviest veins show in the underground workings 

 fre<iuent places where the walls "shut down." Such fillings of clean 

 quartz may vary in width from a few inches to several feet. " Horses," 



I 



•G. F. Becker : Quicksilver Deposits of the Pacific Slope, p. 4()9. 



fit Is not truf thiit every fissure vein holds out to indefinite depth, tliough it is probable that 

 most of the larger veins of the gold-belt will continue to a depth exceeding I he limit of practica- 

 ble exploitation. As a rule, the probable permanence of a fissure vein will bo in direct proportion 

 to its traceable length and to its width. In regions where strong sheeting of tlio rocks has taken 

 place it is quite probable that many of the smaller fissures and joint-planes will pinch out and 

 disappear in depth. Fissuring, after all, is most intense near the surface, and probably compara- 

 tively few of the fissures reach down to deep seated regions; when the rocks become plastic by 

 pressure and heat, or by n suitable relation between the applied stresses, such as must prevail 

 below a certain level, all fissures must cease to exist. The smaller fissures probably received 

 their quartz-filling by communication with the larger ones, which must be regarded as the princi- 

 pal conduits for the solutions. 



XXXII-BiM.i.. firm.. Hoc. Am., Vol. 6. 18'.»4. 



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