226 W. LINDGREN — ^CALIFORNIA GOLD-QUARTZ VEINS. 



at the time the conglomerates were formed. It does not appear easy to 

 separate the earlier deposits from the later, but it is probable that they 

 were neither very numerous nor very rich. 



Again, the eruptive activity of late Tertiary time which was centered 

 along the summit and on the eastern slope of the Sierra Nevada was 

 followed by another period of thermal activity, and another line of gold 

 deposits was formed. This intermittently recurring action confirms von. 

 Richthofen's generalization that a region once metalliferous is always 

 metalliferous. Successive eruptions in such vicinity produce successive 

 mineral deposits, while other eruptive centers are wholly barren of them. 



Differing Types of Gold Deposits. 



It is desirable to eliminate a few deposits of a different type from the 

 prevailing one. Most important among them are the impregnations^^ of 

 which several examples occur in the Sierra Nevada and which may be 

 of two types : First, zones containing grains of iron pyrites disseminated 

 in fresh dynamo-metamorphic amphibolitic schists. These zones are 

 seldom strongly auriferous, but may enrich quartz-veins passing through 

 them, and are apparently similar to the so-called " fahlbands " in crys- 

 talUne schists. These deposits are distinctly older than the principal 

 quartz-veins and contemporaneous with the dynamo-metamorphism 

 which produced the schists from the diabases and other rocks. Second, 

 impregnations of later date forming irregular zones, in which the massive 

 rocks or schists have been decomposed and filled with secondary aurifer- 

 ous sulphides. These deposits are probably contemporaneous with the 

 principal period of vein-filling and only a phase of it, in which the solu- 

 tions, instead of following distinct fissures, permeated whole masses of 

 rocks. The first of these types of impregnation is not of great economic 

 importance, but the second sometimes affords large masses of low grade 



ores.f 



Structural Relations. 



Regarding the structural relations of the normal gold-quartz veins it 

 should first be stated that they are fissure veins, and emphatically not 

 so-called segregated % veins or " lenticular masses " in the auriferous slates. 



* This word is here used in its general sense, and not confined to the filling of interstitial spaces 

 in porous rocks. 



fSee later, under " The alteration of the country-rock," page ^35, line 4 fronn bottom. 



X The term " segregated vein " is not quite clear and has been variously interpreted. A. Phil! Ips 

 evidently considered the only criterion of a segregated vein to be in its parallelism with inclosing 

 slaty or schistose rocks, admitting motion along the walls and filling by foreign material, while 

 R. S. Tarr, in a recently published volume, regards a segregated vein as the result of dynamo- 

 metamorphism and a concentration of material from surrounding rocks, preexisting cavities not 

 being necessarj'. I have used it as meaning more or less lenticular openings in the mass of slates 

 and schists, parallel to strike and dip, produced by longitudinal compression and filled by a sort of 

 lateral secretion or exudation from the surrounding rock. 



