STRUCTURE OF THE FISSURE FILLINGS. 
179 
veins the tellurides are practically confined to the vugs, and this is certainly their 
most characteristic occurrence; but in a few cases they are embedded in the compact 
or solid portion of the vein as well. 
There are numerous variations from what may be regarded as the typical vein. 
Some fissures are completely filled with granular pyrite, as in some of the seams 
of the Doctor-Jackpot lode; some with a crumbling mass of pyrite and fluorite, as 
a vein exposed in the Granite mine, on level 6, north of the shaft; some with a 
hard, fine-grained, dark-purple aggregate of quartz and fluorite, as parts of the Blue 
Bird, Buena Vista, Doctor-Jackpot, and Work veins, and a cross vein in the Findley 
mine, which is in places 15 inches wide. Such fissure fillings, however, are seldom 
productive. Some fissures, like the Howard fiat vein, are in some places much 
wider than the typical veins; but although much ore has come from the Howard 
vein, it has not been obtained from the exceptionally wide and vuggy portions. 
Some fissures, like parts of the Gold Coin and Mary McKinney veins, were originally 
filled with brecciated rock derived from their walls. This crushed material was in 
part cemented and in part metasomatically replaced by gangue minerals and ore. 
The Bobtail vein of the Portland mine is also a mineralized breccia-filled fissure. 
In this case, however, the breccia does not seem to have been derived from the 
walls of the fissure. 
The filling of the Pointer vein, which is in places 2 or 3 inches wide, has 
a peculiar cellular structure, the cavities being lined with small crystals of quartz 
and fluorite. The structure is due in part to the filling of fissures in a sheeted 
zone and in part to a combination of metasomatic replacement and solution of the 
thin slabs of rock between the fissures. 
As regards minor structural details, the veins present few features worthy of 
note. The vein quartz, when seen in thin section under the microscope, rarely if 
ever shows such optical homogeneity as is characteristic of the quartz of the 
Mesozoic gold veins of the Pacific coast. The Cripple Creek quartz has a wavy 
extinction between crossed nicols and shows a pronounced radial optical structure, 
as if each crystal grain were composed of submicroscopic radial fibers. The struc¬ 
ture is suggestive of chalcedony, but is more shadowy, with less regularity and 
sharpness of definition of the constituent fibers. A similar structure is common 
in the quartz of the San Juan region, Colorado. While having some resemblance 
to the well-known strain phenomena produced in crystals by pressure, the structure 
here described can scarcely be explained as a result of strain. The structure is 
particularly well shown in quartz crystals projecting into vugs, as in the Howard 
flat vein, and although the vugs may have been filled with liquid under pressure, 
such hydrostatic pressure could hardly produce strain phenomena. In the Gold 
Coin vein crystals of quartz with this radial optical structure are incrusted with 
chalcedony. The boundary between the two minerals is not sharp, and there 
seems to have been no break between the deposition of quartz and that of 
chalcedony. This suggests that the structure of the optically anomalous quartz 
is also minutely fibrous and in no way due to pressure upon a formed or partially 
formed crystal. It is noted that a lack of optical homogeneity is characteristic 
also of the adularia crystals that project into the vugs of the ore metasomatically 
deposited in the Pikes Peak granite. 
