STRUCTURE OF THE FISSURE FILLINGS. 
177 
the veinlets which contain the tellurides are no thicker than a sheet of paper, as 
may be seen in the Captain stopes in the Portland mine, in the northern part of 
the Mary McKinney mine, in the stopes in gneiss in the Midget and Abe Lincoln 
mine, and in many of the lodes in phonolite dikes. Even such minute fissures 
are not everywhere completely fdled. They are generally planes of easy fracture 
and their walls, when cleft apart, are seen to be covered with a thin drusy coating, 
consisting in most cases of quartz or fluorite, with crystals of calaverite. In the 
Captain stopes the drusy coating is usually dolomite. In the gneiss of the Abe 
Lincoln and Midget mines many of the gold-bearing fractures are so narrow as to 
be visible only by the aid of a lens. As a rule, each little fissure, however, is asso¬ 
ciated with a zone of metasomatic alteration in the gneiss, suggestive in its general 
appearance of the bands of greisen accompanying cassiterite veinlets, such as those 
of Geyer in Saxony. This zone is usually greenish, lacks the brilliant scales of 
biotite elsewhere abundant in the gneiss, and may be an inch or more wide. It is 
thus in most cases a more conspicuous feature than the fissure which it accom¬ 
panies. The veinlets themselves commonly consist of films of pyrite or quartz 
and tellurides, and many of them, as may be seen with a lens, are minutely vuggy. 
The majority of the individual veins in the district range from the width of 
those just described up to 5 or 6 inches, and the vugs have a corresponding range 
in size. The crustified minerals on the fissure walls grow outward somewhat 
unevenly, so that vugs occur irregularly in the veins instead of being uniformly 
arranged along the medial plane. Moreover, some parts of a vein may be solidly 
filled with tellurides and gangue, while other parts show merely thin crusts on the 
walls, as may be seen in the C. K. & N. vein and in some of the flat veins in the Mary 
McKinney mine. In some sheeted zones slabs of country rock became loosened 
when the fissures were opened. Subsequent deposition of vein minerals has cov¬ 
ered these detached fragments with crystalline coatings and cemented them at 
points of contact to the fissure walls, leaving vuggy spaces into which it is possible 
to thrust an arm. Some of the best examples of such gaping sheeted zones were 
seen in the C. K. & N. and Blue Bird mines. Where the Assuring is locally irregular, 
as in the pay shoot of the El Paso vein alongside the phonolite dike (see p. 351), the 
entire ore body may consist of a mass of rock fragments cemented together by 
little vuggy veinlets of quartz or other gangue mineral, carrying tellurides of gold. 
Where the shattering has been more intense and where more or less of the shattered 
rock has been removed in solution, each fragment may be surrounded by an envelope 
of quartz and fluorite, as in a curious expansion of the Elkton lode presently to be 
described. 
Although the vugs are usually small and often minute, a few veins in the dis¬ 
trict contain cavities of notable size. The Howard flat vein, where cut in the 
Ophelia tunnel, has vugs large enough for a man to crawl into, and lined with rough 
crystalline incrustations of quartz and fluorite. The crystals of these minerals 
have evidently in part grown around hollow siliceous pseudomorplis, probably after 
celestite. In one of the cross veins in the Last Dollar mine, on level 12, occurred a 
vug 12 feet long and 2 feet wide. Probably the largest and most remarkable 
cavity in the district was that on level 7 of the Elkton mine, whence issued the 
