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MINERALOGY-THE TELLURIDES. 115 
Its texture is spongy and brittle. In places it is covered by rusty fdms of a ferrugi¬ 
nous compound containing tellurium (Penrose), and this coating often interferes 
with amalgamation. The gold of Cripple Creek is of great purity; sometimes, 
indeed, silver is present only in traces. 
Free gold does not, as a rule, occur in the veins except where set free by oxidation. 
To this general statement there are, however, some exceptions. Bright gold of 
normal characteristics occurred on a seam with roscoelite in the El Paso mine and 
in tetrahedrite in the lower levels of the Doctor-Jackpot mine. Similar gold is 
reported to have occurred in the Laura Lee mine, on Mineral Hill. Examination 
of some telluride ores, supposedly free from oxidation, seems to indicate that a 
very small part of the gold is free; it is, however, very difficult to assert that no 
oxidation has taken place. The beginning of this process is indicated by a thin 
brown film on the telluride crystals. 
The placer gold, chiefly won from the southwest slopes of Mineral Hill, has 
the same characteristics as that from the oxidized zone of the veins. 
Pseudomorphs of this rusty and spongy gold after calaverite and other tel- 
lurides are common. They usually show the normal prismatic form of the calave¬ 
rite and their faces are often warped and shrunken by loss of volume. Fine speci¬ 
mens of these pseudomorphs were seen from the Mercer and Zenobia veins. In 
the latter case their surface was covered by a film of opal. 
Silver .—No native silver is reported from the district, although it might well 
form in places by the oxidation of tetrahedrite, some of which is very rich in this 
metal. Penrose reports a locality “on the hill above the Rosebud mill”® near the 
confluence of Cripple Creek and Arequa Gulch, where the gold contains 40 per cent 
of silver. 
Copper .—Native copper was noted in a specimen from an altered basalt dike 
on the Hillside claim on Tenderfoot Hill. A seam containing partly crystallized 
copper is reported to have been found on Mineral Hill. The mineral might easily 
form by the oxidation of tetrahedrite. 
Tellurium .—Tellurium has been reported from Raven Hill in crystallized form, 
but the find is not authenticated. Its general absence from the veins of the district 
is somewhat remarkable. 
THE TELLURIDES. 
As th£ principal ore minerals of Cripple Creek consist of tellurides, a brief review 
of the various compounds known in nature and belonging to this class may be of 
interest. The principal telluride of Cripple Creek is calaverite, although occurrences 
of sylvanite and petzite are also common. 
The tellurides are minerals with metallic luster, usually soft, the hardness rarely 
exceeding 3; semisectile or brittle. The specific gravity is usually about 9; calave¬ 
rite is the heaviest of the number and reaches 9.4. 
Coloradoite .—Mercuric telluride (HgTe). Massive; iron black; not definitely 
known from Cripple Creek. 
a Penrose, R. A. F., jr., Mining geology of the Cripple Creek district: Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, 
p. 120. 
