\ 
MINES OF BEACON HILL. 359 
GEOLOGICAL FEATURES. 
The workings are nearly all in gneissoid granite, near the contact with the 
main phonolite mass of Beacon Hill. In this respect the ^geological conditions are 
similar to those in the El Paso and C. Iv. & N. mines on the other side of the hill. 
The contact between the 
granite and the phonolite 
is very irregular, as may 
be seen from fig. 41. It 
dips, as a rule, steeply into 
the hill, the average angle 
being probably about 75°. 
In places the granite is 
only slightly brecciated 
near the contact. Else- 
where the two massive 
rocks are separated by as 
much as 15 feet of breccia, 
composed of mingled frag¬ 
ments of phonolite and 
granite. There is usually 
much pyrite along the 
contact, impregnating 
both massive and brec¬ 
ciated rocks, but no ore. 
In the open cut and 
stopes in the Prince 
Albert mine bodies of 
similar breccia are in¬ 
closed in the phonolite. 
It is probable that an 
explosive eruption first 
shattered and pierced the 
granite of Beacon Ilill 
and that the phonolite 
magma subsequently rose 
in the volcanic neck al¬ 
ready partly. filled with 
the phonolite and granite 
fragments produced by 
the explosion. In the 
open cut of the Prince 
Albert the phonolite cuts the breccia with'a well-defined intrusive contact. 
The granite is traversed by several irregular dikes and sheets of phonolite, 
some of which are clearly offshoots from the main phonolite neck or stock. 
Fig. 41.—Sketch plan of level 4, Gold Dollar mine, showing relation of fissures to 
the phonolite plug of Beacon Hill. 
