ransome.] STRUCTURE OF THE LODE ORES. 93 
cles are chiefly argentiferous galena, sometimes a lead sulphobis- 
muthite, and occasionally tetrahedrite. Proustite (ruby silver) was 
noted in the Ben Butler mine, with finely crystalline galena, which 
is erroneously called brittle silver by the miners. 
The gangue of the rich ore of the Camp Bird mine shows several 
interesting variations in its microstructure, and differs from that of 
any other lode known in the quadrangle. A characteristic structure 
and mmeralogical composition is that shown in PI. X, B, where quartz, 
calcite, and fluorite are crystallized together. This is a common 
appearance of the pale-green, finely crystalline aggregate which has 
crystallized concentrically outside of the dark, Wavy ore band shown 
in PI. XII, B. The quartz here shows a distinct tendency toward idio- 
morphic form, and in sections transverse to the prism usually reveals, 
in polarized light, a zonal structure the outer shells of which are 
composed of shadowy sectors differing in their interference tints (PI. 
X, B). These quartz grains contain inclusions of chlorite and a min- 
eral in acicular crystals too small for identification. The inclusions 
are most abundant near the center of the crystal. The fluorite is not 
always present, and the gangue is frequently much more finely crys- 
talline and often shows nests of serieite. This portion of the gangue 
has evidently crystallized in part in open spaces, but it has also 
undoubtedly in part resulted from a metasomatic replacement of the 
San Juan breccia adjacent to or included within the main fissure 
zone. The microscope shows I he free gold to occur in nests of minute 
hackly particles solidly embedded in quartz. It is apparently most 
abundant in the coarsely crystalline, vitreous, radial quartz (see PI. 
XII, B), but also occurs in the quartz just outside of the dark ore band 
already referred to. As seen underthe microscope this ore band con- 
sists of particles of galena, light-yellow sphalerite, and chalcopyrite 
in quartz. The gold was not seen in the dark band itself, but is 
nearly always within a few centimeters of it, usually associated with 
a little galena and sphalerite. 
Although as a rule the ores occupy filled fissures, yet in a few cases 
the gangue in w Inch the ore occurs is composed wholly or in part of 
country rock which has been altered to an aggregate of quartz, and 
usually one or more of the following minerals: Calcite, serieite, kao- 
lin ite, chalcedony, fluorite. Such aggregates generally differ from true 
vein filling in showing under the microscope a much finer crystalliza- 
tion of their constituents into a mosaic which is more or less cloudy 
when seen between crossed nicols. The grains of quartz seldom 
exhibit the clear, definite boundaries seen in vein filling, such as is 
shown in PI. IX. Moreover, the serieite and calcite are often 
grouped in areas whose form is that of former feldspathic pheno- 
crysts. This structure, however, will be more fully described in the 
section on the metamorphism of the country rock. 
