164 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
The Bobtail vein is a sheeted zone that lor at least a part of its course follows 
an older fissure in the granite. This fissure is filled with breccia composed chiefly of 
granitic material. It is probable, however, that this breccia is formed of particles 
that were carried into an open fissure at the time of eruptive activity rather than 
to trituration of the fissure walls by great movement. 
Parts of the sheeted zone of the Mary McKinney vein contain a dominant 
fissure up to about. 5 inches in width, which is fdled with fragments of phonolite 
partially replaced by roscoelite and fluorite. In this case the movement along the 
fissure zone was sufficient, at least locally, to brecciate the sheeted phonolite. In 
the Wardel vein, in the Anchoria-Leland mine, the thinly sheeted breccia of the 
medial part, of the lode is in some places shattered, showing movement along the 
fissure zone. 
The Doctor-Jackpot lode exhibits slight slickensides along portions of its regular 
foot-wall fissures. The displacement, however, is probably slight, as great move¬ 
ment could hardly fail to brecciate the thin sheets of the fissure zone. There has 
evidently been some movement along the Mattie D. vein of the Doctor-Jackpot 
mine, as shown by brecciation along the foot-wall fissure. The Cardinal vein, a 
crushed zone 1 foot wide in granite, shows well-defined horizontal slickensiding 
along the hanging wall on the 200-foot level. In parts of the Gold Coin vein, near 
the north end of the mine, the closely sheeted granite grades into brecciated material 
3 or 4 inches wide. The movement necessary to effect this brecciation of the thin 
laminae of rock was probably slight and was certainly local. 
The structures thus far described are those associated with lodes traversing 
large masses of uniform rock, such as breccia, latite-phonolite, syenite, or granite. 
Many of the fissures, however, pass from one rock into another or follow dikes and 
it is necessary to briefly note the structural details connected with such associations. 
The phonolite dikes have a general tendency, as is revealed by weathering, to 
split into thin slabs roughly parallel to the dike walls. Fissuring along the line of 
such a dike merely accentuates this inherent structure, and forms a well-defined 
sheeted zone which may involve the whole width of the dike or may constitute a 
narrow zone along one or both w r alls. A considerable number of the lodes in the 
district are wholly or in part sheeted phonolite dikes. As examples, may be men¬ 
tioned the Independence and Port land veins and the Cobb vein of the Moon-Anchor 
mine. A number of other lodes, such as the Apex vein of the Ajax mine, lie in the 
breccia or granite alongside phonolite dikes. 
In those cases where a fissure zone and a dike or mass of phonolite intersect 
nearly at a right angle, the platy parting of the phonolite tends to destroy the regu¬ 
larity of the sheeted zone. Thus the Walter vein of the Elkton mine, which in the 
breccia is a regular sheeted zone, becomes very irregular in the masses of phonolite 
described on page 333, and splits up into fissures running in various directions, many 
of them being nearly horizontal. The Spur and Coin veins of the Gold Coin mine 
also lose their regularity and become indistinct in an intrusive phonolite sheet which 
they cross near level 6. In the Isabella mine the Cheyenne vein, below level 10, 
and the Empire No. 2 vein, on level 11, become small and barren seams when they 
enter phonolite. 
