STRUCTURE OF THE GOLD DEPOSITS. 
159 
INFLUENCE OF COUNTRY ROCK. 
Fissures are most abundant in the breccia and in the Pikes Peak granite adja¬ 
cent to the volcanic neck. Productive lodes occur, however, in all the other rocks 
within or near the neck, with the possible exception of the schist. The latite- 
plionolite, on the whole, seems less favorable than breccia to the development of 
ore-bearing fissures. Many fissures in breccia, such as the Captain veins of the 
Portland mine, terminate at or near the contact with masses of latite-phonolite, 
though others, like the Pharmacist, the Isabella system, and the Vindicator system, 
pass from breccia into latite-phonolite without noticeable change. 
Owing to their tendency to develop a platy parting parallel to their walls, 
dikes of phonolite or “basalt” when they happen to lie in the general direction of 
Assuring, frequently become sheeted zones. As examples of such occurrences may be 
mentioned parts of the Independence vein, the Portland vein, the Montana vein 
of the Gold Coin mine, the Apex vein of the Ajax mine, and the Cobb vein of the 
Moon-Anchor mine. In the lower levels of the Hull City mine one of the veins 
follows a dike of latite-phonolite, 10 feet wide, in breccia. This is the only instance 
observed in which a dike of this rock contained ore. 
When, however, a dike lies across the planes of Assuring, the Assures may con¬ 
tinue through the phonolite, but more often they become indistinct or irregular in 
that rock, as will be described when an account is given of the detailed structural 
features of the Assures. 
The general behavior of a Assure zone at a contact between two rocks depends 
largely upon the angle at which the plane of the lode meets the plane of the contact. 
If the two are nearly perpendicular the Assures may stop at the contact or they 
may cut through it without deAection. Instances of a Assure zone ending abruptly 
at a contact between two rocks are not common in the district, though there is often 
a marked change in the character of the ore at such points. The Diamond vein of 
the Portland mine, which is in granite, ends against the breccia, but the original 
relation has been obscured by slight faulting along the contact. The Gold Coin- 
Dead Pine Assure zone in granite has been followed to the contact with the breccia, 
where it apparently terminates. The No. 6 vein of Stratton’s Independence mine 
is known only in the granite, and the Emerson vein of the same mine is known only 
in the breccia. It could not be determined, however, at the time of visit, whether 
these Assures actually end at the contact. The Independence and Bobtail veins, 
on the other hand, pass from granite to breccia without deAection, though the 
Independence, like the No. 2 vein of the Portland mine, coincides with a portion 
of the contact that happens to lie in its course. The C. K. & N. Assure zone, which is 
regular and persistent in the granite, ends on the west against the phonolite sill of 
the Old Gold mine. 
When a Assure zone and contact meet at an acute angle the Assures may change 
their course and follow the contact, particularly if it is the wall of a dike, as in the 
case of the C. Iv. & N. vein and the El Paso phonolite dike, described on pages 
351-352, 356. In some instances, such as that of the El Paso vein (Ag. 39, p. 350), 
the deAection is only partial, the Assures Anally crossing the dike and continuing 
pn the other side with their original course. 
