January 19, 1894. , 
a width or thirty feet or more. This mine is a good 
example of a great ‘‘sheeted” fissure—the sheets or 
plates varying from a few inches to two or three feet in 
thickness and separated from each other by nearly ver- 
tical quartz seams often rich in gold. ‘There is also in 
this mine, a rich pay streak of vesicular quartz, wad 
and kaolin, which perhaps occupies what may be con- 
sidered the ‘‘main fissure.’”’ The little quartz seams 
are the ‘‘minor fracture cracks” filled with secondary 
auriferous quartz, and the ‘‘sheets” or ‘‘plates’’ the 
country rock mineralized and changed into low grade 
ore. Seams and partings of kaolin (the tale or china- 
clay of the miners), as well as the selvages referred to, 
are common. Such seams are often followed down- 
ward, through the ‘‘slide” or ‘‘wash,’’ which is gener- 
ally the top beds decomposed in situ, and lead to good 
veins in thesolid formation. Much of the ore is coated 
and impregnated with fine, white,. auriferous iron- 
pyrites, associated with graphic tellurium (sylvanite), 
forming a rich and valuable ore. Some of this sylvanite 
ore is very beautiful as well as valuable—the surfaces 
being stained a rich blue verging on purple, by fluoric 
acid, and glistening with fine silvery white bars of syl- 
vanite. 
Few valuable minerals besides gold are found in the 
district. . Silver is very seldom met with—the trifling 
amounts found in the ore being of no commercial im- 
portance. Copper is still rarer. Limonite is met with 
aS a vein stone, and, in two or three instances, as a.cap 
to the lodes. It seems to carry little gold, as a rule. 
Earthy oxide of manganese (wad) occurs in the veins 
with kaolin. A quartz peculiar to the district is the 
granular, sugary, massive quartz, often colored blue or 
purple by fluoric acid. This rock or ore contains a 
little orthoclase in grains, and is probably a felstone 
again altered by the mineral solutions circulating through 
the veins. In many mines itis rich in gold. Galena 
is very rare, but it is occasionally seen in theform of 
very small grains in vein stones. In one mine, fine 
anglesite crystals are found in geodes in a vein rock 
containing small grains of galena. They are small, 
tabular, white crystals of adamantine lustre, and result, 
doubtless, from the decomposition of the galena. Most 
of the vein matter is simply altered country rock—the 
kaolin resulting from the decomposition of the feldspar. 
The vein quartz is the silica which was separated from 
the silicate (orthoclase) and held in solution in the 
alkaline waters at the time of the kaolinization of the 
fieldspar. The iron and gold may have been derived 
from the iron-bearing micas (perhaps directly in some 
cases, or from pyrites which were themselves derived 
from the micas) and held in solution in the same alka- 
line waters that held the silica. These alkaline waters, 
circulating through the fissures, impregnated the walls 
and plates, and filled the cracks with auriferous quartz, 
together with the other minerals of the solutions. Sur- 
face water containing organic matter may have aided 
in precipitating the minerals. One of the most inter- 
esting and peculiar vein-stones here found is the 
variously colored jasper occurring in considerable quan- 
tities in the ‘Victor Mine” and to some extentin others. 
In the ‘‘Victor” the bright-colored cryptocrystalline 
jaspers are said to be very rich in gold, while the dark- 
brown earthy-looking varieties carry little or no value. 
This is very surprising, but a visitor will find very little 
of the first named variety ‘‘on the dump,” while in the 
ore house he will find men carefully sorting and sacking 
ore for the smelters, and here he can see large quanti- 
ties of jasper, some striped in shades of red, white and 
light brown, all of it going into the ore sacks. Sulphate 
of baryta (heavy spar) was noticed in one mine—asso- 
SCIENCE, 33 
ciated with crystals of pyrite. Celestite, tourmaline 
and a few other interesting minerals occur, but are very 
rare. The mineral veins are, as a rule, singularly free 
from minerals valuable only to the mineralogist. 
While the Cripple Creek mining district must be con- 
sidered a typical granitic region, the two isolated erup- 
tives heretofore mentioned are interesting to geologists 
and petrologists. The Black Dyke traversing the 
northwestern part of the district is at least thirty feet 
wide. It is questionable whether it reached the surface 
at any point, until erosion carried away the overlying 
granites andschists. Where partially excavated in the 
town, it lies in a belt of black, friable mica schist, but 
was not sufficiently uncovered to show the contact on 
either side. Further northwest on a spur ofthe granite 
ridge, which bounds the town on the west, this dyke is 
seen cutting through the granite, and some local met- 
amorphism is apparent at the contact. Here it seems 
to have thrown out an arm; or rather, it looks as if, un- 
able to force its way higher, the pressure from below 
was sufficient to force the granite beds upward a little, 
forming a space into which the molten or plastic matter 
found its way horizontally. Underlying this arm or 
sheet, between it and the granite, is a horizontal vein 
of white, massive, quartz, six inches in thickness close 
to the dyke, but gradually widening to two feet or more, 
forty feet away. The eruptive sheet becomes thinner 
as the quartz vein grows thicker, so that, taken togeth- 
er, they measure about the same throughout their 
course. The granite, which doubtless overlaid both, 
has been eroded away. Some prospector has opened 
the quartz vein by an open cut to the dyke, and tun- 
neled some feet into the dyke itself. The quartz vein, 
where covered by the eruptive sheet, is strongly im- 
pregnated with copper pyrites, and coated blue and 
green by copper carbonates. Forty feet from the dyke 
the eruptive sheet comes to an end, and the quartz be- 
comes white, losing all trace of copper. ‘This is the 
only place in the district proper where evidence of cop- 
per was noticed, and it is clear, in this case, that the 
copper was derived directly from the eruptive rock. 
Macroscopically this dyke rock is a basic, compact, 
almost aphanitic, rock, very dark gray or almost black in 
color and showing minute, metallic grains disseminated 
throughout the mass. A polished surface, with the aid 
of a pocket lens, shows fine feathery flakes of a plagio- 
clase feldspar ; the interstitial spaces being filled with a 
black or very dark green mineral (amphibole or 
pyroxene). This rock resists erosion well but weathers 
brown, slightly, on exposed surfaces. Its age is hypo- 
thethical, and microscopical study will be necessary to 
determine its petrological name. 
The eruptive forming ‘‘Bull Cliffs” is of an entirely 
different character, and is known in the camp as 
“‘trachyte.” It is a micro-crystalline rock containing 
glassy crystals (sanidine). A part of the deposit con- 
tains considerable hornblende in grains and prisms. 
The upper beds, however, contain little or no horn- 
blende and are of a fine mottled greenish-gray and 
white appearance. This rock splits into slabs, often as 
thin as one-eighth of aninch with smooth surfaces. It 
resists erosion much better than the underlying horn- 
blende-bearing rock. The latter disintegrates readily; 
exposed surfaces have a hackly appearance, and are 
spotted white with a decomposition product of the rock. 
The hackly weathered surfaces are often so friable as 
to crumble off in handling. Large blocks ring almost 
like steel when struck with thehammer. Hydrochloric 
acid eats out a considerable part of the rock (probably 
the nepheline). This eruptive does not seem to occur 
as a dyke, but rather as an isolated, massive eruption 
