228 W. LINDGREN — CALIFORNIA GOLD-QUARTZ VEINS. 



of the greater and minor strains to which the Sierra Nevada has been 

 subjected, and a study of the former will, to a considerable degree, illus- 

 trate the latter, which have certainly varied in intensity and direction 

 from point to point. Thus, to pick out a few illustrating examples, the 

 veins of Ophir, Placer county, consist of two principal systems, one set 

 of veins running west-northwest and dipping south, while the other has 

 a west-southwest strike and southerly dip, both cutting the surrounding 

 schists obliquely to their strike and dip. At Grass Valley and Nevada 

 City there is one system with a general northerly direction and dipping 

 either east or west ; another system courses east and west and dips north 

 or south at varying angles. Ttie surrounding rocks are here mostly 

 massive. The veins in the vicinity of Sierra Buttes, Sierra county, show 

 the greatest divergencies in strike and dip. Equally variable are the 

 veins about Sonora, Tuolumne county. 



The force producing these fissures appears in most cases to have been 

 a compressive stress acting at an angle more or less oblique to the hori- 

 zontal. In some cases this force produced one large and prominent 

 fracture, but far more commonly one or several series of fractures, or a 

 sheeting* of the country-rock along which the auriferous solutions could 

 circulate. Along the larger fissures considerable movement has taken 

 place, but when the country-rock has been sheeted the motion along the 

 individual joints has probably not been very great. In many cases, 

 when the direction of the movement could be proved, it has been found 

 that a relative upward movement of the hanging wall has taken place. 

 The force did not produce a single, sudden and catastrophic movement ; 

 on the contrary, it continued for long time, resulting in repeated dislo- 

 cations, as proved by the reopening and refilling of some veins and by a 

 sheeting of some veins, producing what is usually described as " ribbon 

 rock." Recemented quartz-breccias are also of common occurrence. 



I should here like to mention one misleading circumstance relating to 

 parallelism of vein and country-rock. When larger fissures are opened 

 in massive rocks it is not at all uncommon to find the immediately ad- 

 joining wall-rock converted entirely locally into schists parallel to the 

 fissure, under the influence of the enormous shearing stress to which it 

 has been subjected. Such veins would have the appearance of cropping 

 in preexisting schist-masses, and of parallelism in strike and dip with 

 these. The conclusion to be derived from the relation of the veins to 

 the larger, regionally metamorphosed schist-masses is that the schistose 

 structure antedates the formation of the vein fissures ; and that the forces 



* The relation of the forces and the sheeting has been discussed by Mr G. F. Beclvor : Bull. 

 Geol. Soe. Am., vol. 4, p. 13. See G. F. Becker, "Geology of the Comstoclc Lode," Mon. Ill, U. S. 

 Geol. Survey, p. 182, and S. F. Emmons, " Structural Relations of Ore Deposits," Trans. Am. Inst. 

 Min. Eng., vol. xvi, p. 814. 



