80 
ALASKAN MINERAL RESOURCES IN 1904. 
[BULL. 259. 
the microscope shows the presence of minute fracturing between the 
veins visible to the naked eye. The veins are usually closely spaced, 
and an estimate based on a study of all the mine workings indicates 
that infiltrated materials make up nearly one-fifth of the mass of the ore. 
The boundaries of the veinlets against the inclosing rock are some- 
times distinct, but in many cases there is an apparent gradation from 
the vein matter into the altered diorite. When the amount of intro- 
duced minerals is large in proportion to the mass of the matrix, in 
small specimens it is often difficult to distinguish the vein stuff from 
the rock, though in large fragments or on the stope faces, the general 
extent of the different portions of the ore material is exhibited. The 
microscope shows that the merging of the interstitial veinlets with the 
rock which they cut is due to penetration of the latter by calcite, 
which is intercrystallized with secondary albite, formed at the expense 
of the original feldspar. 
Veinlets traverse the rock in different directions, but the greater 
part of the filling occurs in fissure-like openings constituting two 
well-marked systems. One set of 
fractures strikes and dips approxi- 
'f mately with the structure of the inJ 
closing slates; the other, which is 
the more prominent, strikes some- 
what obliquely to the structure of the 
country rock and dips in the oppo- 
site direction — that is, toward the 
southwest. 
In places where the mineralized 
dikes are narrow, the set of fissures 
parallel to the country rock structure 
usually diminishes in importance] 
and often only the cross fractures have been developed. This may bd 
explained upon the supposition that the tendency to motion parallel: 
to the walls of the intrusions was taken up outside of the massive rocla 
in the slates, while the transverse strain affected both the slate and thd 
intrusive rock, the latter being specially susceptible to cross fracture] 
because of its small mass and brittle nature. Cross fractures, tilled 
with vein stuff and limited to a narrow dike in the slates, may be seen 
to good advantage at the east end of the Ready Bullion pit, near the 
southernmost outcrop of the diorite (fig. 4). Throughout the mines ii 
is the rule that all transverse gash veins stop at the walls of the diorite 
and while there are a few exceptions the quartz seldom penetrates the 
country rock to any great distance, and when it does it diminishes 
rapidly in thickness. However, this is not always due to the nonper 
sistence of the fissures, for they may be frequently observed continu 
ing from the diorite into the slate in the form of well-marked joints. 
Fig. 4.— Dike of albite-diorite in open cut of 
Ready Bullion mine. 
