7 2 
GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
Feldspar .—Orthoclase is common in phenocrysts from 0.05 lip to more than 
1 cm. in size. Columnar crystals parallel to the clino-axis have fairly good basal 
and pinacoidal faces. Tablets parallel to the clinopinacoid are usually of poor 
crystallographic outline, though the base is sometimes well developed. Carlsbad 
and occasionally Baveno twinning is seen. Cleavage appears to be less perfect 
than in the phonolites. The angle of the position of extinction with the basal 
cleavage sometimes indicates soda-orthoclase. The usual product of decomposition 
is kaolin, while sericite forms near mineral veins. 
Several species of triclinic feldspar occur as phenocrysts. Microcline is present 
sparingly in a few rocks. Microperthite, with its characteristic patchy and flaky 
extinction, is frequently seen. A few individuals may be referable to anorthoclase, 
but not with certainty. The most important phenocrysts of these rocks belong 
to the ordinary plagioclase group, and range from albite to labradorite inclusive. 
The variety which is by far the most plentiful appears to correspond to a compo¬ 
sition between albite and oligoclase, while species more calcic than oligoclase are 
uncommon. Calculation from analyses shows that only about 1 per cent of the 
total lime of the rock enters into the plagioclase. The phenocrysts are usually 
tabular parallel to the brachypinacoid and have rather imperfect crystallographic 
boundaries. In some cases, however, they occur as stout prisms bounded by 
well-developed pinacoids, base and macrodome. Albite twinning is almost invariably 
present, but because of the frequency with which clinopinacoidal sections showing 
no twinning are met, the importance of plagioclase is at first underestimated. 
Carlsbad and pericline twinning sometimes also occur, and in one case albite and 
Baveno twinning occur in the same individual. Zonal structure is very common. 
The change is gradual and uniform and follows the usual order of solidification. 
Particularly in those rocks with abundant feldspar phenocrysts and subordinate 
groundmass it is common to find the plagioclase bounded by a zone filled with 
minute gas bubbles and opaque inclusions, and then surrounded by a narrow rim 
of clear orthoclase. This orthoclase mantle appears to develop more on the pina¬ 
coids than on the base, sometimes leaving the latter face wholly uncovered. Occa¬ 
sionally a micropegmatitic or perthitic intergrowth of plagioclase and orthoclase 
is observed. Small brownish isotropic patches of irregular form occur without 
definite distribution through some of the larger individuals and represent inclusions 
of glass. Microlites or small irregular grains of pyroxene are sometimes also 
included in the plagioclase. 
Kaolin is the chief product of 'weathering. Epidote results under certain 
conditions from alteration of the calcic feldspars. A very interesting decomposition 
results in a pseudomorphic change of the feldspar into a colorless isotropic material 
of low refractive index. The transformation begins along cracks and cleavages, 
forming a meshwork around unaltered kernels, and in most specimens the 
plagioclase has been entirely replaced. This decomposition has been noted only 
in those plagioclase individuals which are surrounded by the mantle of orthoclase, 
and in every case the monoclinic feldspar remains clear and unaltered. When a 
thin section containing this decomposition material was warmed with dilute 
hydrochloric acid and then treated with fuchsine, the area within the orthoclase 
rims absorbed the dye. This test strengthens the conclusion drawn from the 
