Gilpin observer., 1913-07-03

From this investigation, it appears
that the entire tunnel will be in pre-
Cambrian schii5t and gneiss. This
Is the rock commonly called old
granite by our prospectors. 1t be-
longs ITO a group of rocks which are
the oldest known ON the face of
the earth. They were formed be-
fore the time of hmestones, states,
clay, shales and the like, which are
the bane of tunnel builders. These
old crystalline rocks are very homo
geneous. There are literally hun
dreds of miles of mine workings in
this same formation in Boulder, Gil
pin and Clear Creek counties. We
know all about its qualities from
every angle. There lis no guesswcrk
about it. We know that some parts
are hard to break and some parts
easy, that lit all stands well, except
where decompcsed by mineral action,
and that softened zones are seldom
wide.
When vein containing pyrite
begins to oxidize, there is formed
sulphate of iron. which, when dissolv-
ed in circulating waters, bring about
the kaolinization OF the surrounding
granite rocks. This action is pure
ly one of surface action and does
not extend to great depths, so that
the few instances of soft heavy dirt
DIN the upper levels of the mines in
the above named counties have little
bearing on the Moffat tunnel enter
prise.
THE deep mines of Central City
show that the rocks near the veins
are mere or less softened down to
lowest depths explored (2,20O feet).
The Newhouse tunnel, however, cuts
dozens Of these veins and but little
timber or other Inning is used after a
depth of 500 feet from the surface
is attained, proving thereby that the
softening at depth is not sufficient
to cause caving. NJ
'The Moffat tunnel section shows
no veins and only one dixie directly
on the tunnel tangent, but little
to one side there are exposed three
veins, which, if persistent, will cross
the tunnel about one and one-half
miles from the east portal and at a
depth of about LOO feet. At this
depth it is very unlikely that there
will be any kaohnization of trouble
some softening of the rock.
About mile north Of the tunnel
tangent on the eastern slope OF the
range, there is quite a showing Of
eruptive rock (porphyry) and attend
ant naturalization. Several people
believe that they have discovered
important mines of copper, silver
and gold in this neighborhood. On
the south side of the tunnel, / have
found veins that carried low grade
values in gold. So it will be seen
that there is some small chance of
striking ore in driving the tunnel.
LAS result of a careful study of
the geology of the Moffat tunnel sec
ton 1t appears that the entire oun-
mel will be in pre-Cambrian schtst
and gneiss, which, although banded
in structural detail, is remarkably
homogeneous en- masse. This rock
has some very hard bands i'm it, that
will require the best steel, equip
merit and powder to penetrate rapid
iy; it has. however, no soft or por
GUS zones, and taken as whole, 1t
is a very desirable formation in
which to tunnel. The three vedns
and one dixie indicate two zones of
metamorphism and possibly of miner
alization. It is not probable that
either zone will be soft enough to
need any lining or timbering, but.
Il so, the bad ground wSl be con-
fined to narrow strips.