stuuctt^kal relations. 227 



It is everywhere plain and evident that the fissures have heen broken 

 open subsequently to the nietamorphism of tlie rocks. These post- 

 Jurassic and post-granitic quartz-veins form the hitest chapter in the 

 Mesozoic revolution in the Sierra Nevada. 



Neither Whitney nor von Richthofen commit themselves to an expres- 

 sion of the " segregated " nature of the veins. A. Phillips, in his book on 

 mineral deposits, mentions tlicir aflinity to iissure veins, although classing: 

 tliem as " segregated veins." All these writers, however, state that the 

 veins nearly always conform in strike and dip to the inclosing slates. 

 This has evidently led the autliors of recent text books to class the Cali- 

 fornia veins as "segregated." Thus Professor J. F. Kemp, in his "Ore 

 deposits of the United States," classes them as such with some doubt, 

 wliile Professor P. S. Tarr, in his '' Economic geology of the United States," 

 thinks that ''in spite of the recent observations (by H. W. Fairbanks) it 

 still seems as though these quartz-veins must be of segregation origin." 



Quartz-veins like tliose Professor Tarr has in mind, formed by a sort 

 of dynamo-metamorphic process, I am quite sure do not exist in the 

 gold-belt. The somewhat auriferous " fahlbands " in certain amphibolites 

 appn)ach nearest his concei)tion. I am by no means })rei)ared to deny, 

 however, tluit tliere may be some minor ore-bodies deposited in openings 

 in tlie slate from silicious soUitions derived from the immediately sur- 

 rounding rocks, but if they occur, they are surely exceptions to the 

 general rule. In altered quartzose slates nodules and lenses of quartz 

 seemingly of such origin frequently occur on a small scale. 



This rule of '" i^arallelism with inclosing slates" must unquestionably 

 be rejected in a general description of the veins. It should first be 

 l»ointed out that a very large number of veins, especially in the northern 

 I)art of the gold-belt, from Placer to Butte county, do not occur in slates 

 or schists, but in massive rocks, such as diabase, granodiorite or gabbro, 

 and among these a predominating direction of dip and strike does not 

 exist. In slates and schists the veins often strike about parallel to the 

 slaty cleavage — that is, northerly or northwesterly — l)ut other directions 

 are nearly as common. Only very exceptionally is there a strict paral- 

 lelism in both strike and dip. The great mother-lode, for instance, is 

 parallel to the strike of inclosing rocks, but differs not inconsiderably 

 fn)m them in dip. Its character of fissure vein is clear and uncpiestion- 

 able,and has been justly insisted upon by II. W. Fairbanks.-'^ All direc- 

 tions and all dips are in fact represented among the California (piartz- 

 veins, only dij)S below 20° and above 70° are comparatively rare. A 

 general rule for strike and dip cannot be given; different laws guide 

 them in different mining districts. The <piartz-veins are the ex})ression 



•Tenth Ann. Rep. Stnte Mlnernlogi.st of Culifoiniti, 189<), p. 80. 



