January 19, 1894 | 
SCIENCE: 
PusiisHep By N. D. C. HODGES, 874 Broapway, New York. 
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NOTES ON THE GEOLOGY OF THE GOLD 
FIELD OF CRIPPLE CREEK, COLORADO. 
BY H. L. M’CARN, DENVER, COLO. 
THE granites surrounding the mineral district of Crip- 
ple Creek are of the ordinary types common to the 
Front Range. They vary from moderately fine to 
coarse grained in texture. Bedding structure is gen- 
erally rather indistinct, but in places is strongly 
marked, and the rock may be more correctly styled 
gneiss. The quartz in these granites is either of a 
vitreous translucent or milky variety; the feldspar 
(orthoclase) usually pink. Buiotite is the common mica, 
but other micas are met with, especially close to the 
niineral belt; one of these varieties being a black mica 
with splendent lustre, sub-translucent and dark green 
by transmitted light. It is strongly iron-bearing, very 
easily fusible to a black globule and is undoubtedly 
closely allied to, if not identical with, lepidomelaur. 
In the granite ridge bounding the town of Cripple 
Creek on the west, are seen veins of amorphous, white 
or rose quartz, often six inches or more in thickness, 
apparently dispersed without relation to bedding struc- 
ture, or order of any kind. Similar veins consisting 
wholly of pink feldspar occur in the same manner. 
Nests of large tabular prisms of a black mica (lepi- 
domelaur) are also met with. Much of the granite 
here is of a very coarse texture, the various constitu- 
ents being often an inch or more in diameter. These 
veins of quartz and feldspar, the nests of mica, as well 
as the very coarse textured granites, may all be attrib- 
uted to a common cause, and have every appearance of 
having been formed contemporaneously with the consol- 
idation of the containing granite. Tothe east of this 
ridge, in the town, as well as in other parts of the dis- 
trict, there occur ‘‘endogenous”’ veins of quartz, associated 
with parallel veins of feldspar, which are two or three 
feet in width and may be traced some distance in ap- 
proximately straight courses. These veins are proba- 
bly of secondary origin—concretionary veins—due to 
causes similar to those to which the various alteration 
belts of rock of the mineral area may be attributed. 
From the normal granites surrounding the district, 
towards the centre, occur innumerable phases of altered 
granitic rocks. Schists, aplites, felstones, conglomer- 
ates and breccias abound in numberless varieties, and 
transition rocks have been observed linking together 
rocks of very different appearance. Although this 
SCIENCE. 31 
region is laid down on Hayden’s maps as ‘‘eruptive,”’ 
no trile eruptives were seen in the mineral belt proper, 
with the exceptions of a black magnetic dyke in the 
town, and the rock composing what is known as ‘‘Bull 
Cliffs.” These eruptives will be discussed further on. 
To give distinctive names to the various altered rocks 
of the district would be as difficult a task as attempting 
to name the shades from blue to green. Some sort of 
a classification of the most marked types should, how- 
ever, be made. The nomenclature in use by the miners 
is confusing in the extreme and renders an attempted 
description of the country rock of a mine unintelligible. 
Granite, schist, porphyry, quartzite and trachyte are 
the miners’ rocks. The term granite is applied to the 
binary granites as well as to the normal granites, if the 
tock is coarse-grained and the feldspar still pinkish, but 
if the feldspar is white, some call it ‘‘porphyry,’’ while 
others speak of it as ‘‘quartzite,” the choice of terms 
depending, probably, on their individual preference for 
the one or the other as a country rock for their claims. 
These micaless granitic rocks are from course to fine 
granular in texture—some even passing into a micro- 
erystalline aggregate. In color they range from white, 
through bluish-gray and gray, to yellow or brown. 
Some are pyritous, some mellose, in a fine grained 
magma, porphyritic crystals or blotches of white feld- 
spar; while others have a leached-out appearance and 
are often colored yellowish or brownish by iron oxides. 
Curiously, this last-mentioned type is the one universal- 
ly called ‘‘porphyry” by the miners. It composes the 
walls of many of the best mines (especially near the 
surface at depth passing into a grayish, often pyritous 
felstone) and is a favorite country rock. In the veins it 
often forms a veinstone, seamed with secondary quartz 
carrying gold. This so-called ‘‘porphyry”’ is sometimes 
beautifully zoned with concentric rings of brown or yel- 
low, due to the oxidation outward of iron salts—the 
rusty bands giving the rock a riband appearance. The 
interior of such rocks is generally found to be a fine 
granular, gray material somewhat resembling sandstone, 
and often containing small grains of white iron pyrites. 
The micaless granites, where coarse in texture, might 
be termed ‘‘aplites,” the fine-grained varieties ‘‘fel- 
stones.” By prefixing descriptive adjectives, such 
terms as porphyritic gray felstone, pyritous felstone, 
yellow or brown felstone, etc., etc., would convey some 
idea of the character of the rock under discussion. 
There are two distinct mica schists in the camp. In 
one the mica is a black iron-bearing variety, the rock 
often appearing to be a disintegrated granite with schis- 
tose structure and very friable. This rock is considered 
an unfavorable country rock. The other schist is a 
tough, laminated, white rock, the mica being in large 
leaves of glistening, silvery-white muscovite (or a 
hydrous alteration variety of muscovite). This latter 
rock occurs in bands through the heart of the mineral 
area and in one or two instances was seen apparently 
bedded with beds of coarse granular, white ‘‘aplite” ly- 
ing conformably upon its dipping surfaces. Some good 
mines are found in this rock, or rather on its contact 
with other rocks. The ‘‘Bull Cliffs” eruptive is locally 
called trachyte. The term is not, however, confined to 
this eruptive, but is given to many dark varieties of 
rock, of vayious structure, some of which bear a strong 
eruptive appearance in hand specimens. Some of these 
rocks are hard and compact, ring when struck with a 
hammer, and often contain small grains or prisms of 
hornblende. Theserocks are usually in dark shades of 
gray and are occasionally exquisitely traced with imita- 
tive figures resembling trees, ferns, etc., due, doubtless, 
to a saturation of the rock with water, which, acting on 
