STRUCTURE OF THE FISSURE FILLINGS. 
181 
The quartz is apt to vary considerably in texture in a single specimen, ranging 
from rock of fine grain, but with still distinctly visible partly idiomorphic individ¬ 
uals, to rather coarse vein quartz (PI. XVII, C ). The pyrite crystals are usually 
inclosed in quartz; rarely in fluorite. 
Fluid inclusions are abundant and often show moving bubbles which do not 
disappear or change form upon heating to +35° C. and above. The fluid is, there¬ 
fore, in all probability an aqueous solution. The inclusions are apt to have 
extremely irregular forms and often contain some solid material besides the fluid 
and the bubble, but this material is not crystallized; it forms irregular masses and 
adhesion to the walls. 
The so-called “purple quartz” or “massive fluorite” of some of the veins is 
usually a mixture of quartz, fluorite, and dolomite in various proportions. It is 
deep purple and has a fine, evenly granular texture. 
The microscope shows that the dominant mineral is 
generally quartz in allotriomorphic grains. In¬ 
cluded in the quartz are small cubes of fluorite 
and in many cases rhombohedrons of dolomite 
(PI. XVII, A). The fluorite, as is usual in the 
darker varieties of this mineral, is very unevenly 
colored, the deep-purple pigment occurring in little 
flecks here and there in the cubes. On the whole, 
thin sections show much less purple than the deep 
color of the material, as seen in masses, would sug¬ 
gest. In most cases the quartz-fluorite aggregate 
is purely a fissure filling. In the Elkton mine, how¬ 
ever, on level 7, granular fluorite with some quartz 
has metasomatically replaced breccia. Thin sec¬ 
tions of this material show the same uneven colora¬ 
tion, the most deeply colored cubes or grains being 
those which are in contact with the breccia along 
the irregular surface of replacement. At a little 
distance from this contact the fluorite as seen in 
thin section is nearly colorless. This suggests that 
the fluorite grains may in some cases naturally lose 
their deep color which presumably is due to included 
organic material. 
A rather unusual vein filling was noted at one 
point in the C. Iv. & N. vein. The principal fissure 
is here an inch wide and for the most part is only 
partly filled with vuggy quartz and calaverite. At 
one place, however, the vein is filled with a fine-grained dark-gray mass about 3 
inches long, which appears to have been originally a fragment of rock wedged 
•into the fissure. This fragment has been metasomatically altered to an aggregate 
of quartz and minute crystals of pyrite, with here and there a speck of calaverite. 
Tn crusting the ends of the fragment and the walls of the fissure is a layer of quartz 
and calaverite of varying width. Coating this and almost enveloping some of the 
Fig. 13.—Local structure in C. Iv. & N. vein. 
Walls consist of reddish granite containing 
pyrite in cracks perpendicular to vein. In¬ 
closed fragment of altered rock in open fis¬ 
sure and crusted by calaverite and quartz 
covered by radial pyrite. On top of the 
latter comb quartz. 1. Fragment of altered 
rock wedged in fissure. 2. Minutely fis¬ 
sured granite with pyrite. 3. Calaverite. 
4. Earlier generation of quartz. 5. Radial 
pyrite. 6. Later generation of quartz. 
