92 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
orthoclase. Twinning, though not always present, usually follows the albite law 
and sometimes the Carlsbad law also. Pericline twinning is rarely seen. In some 
cases small patches of turbid glass are inclosed in zonal arrangement by' the feldspar 
phenocrysts. Orthoclase is not ver} T abundant, occurring, with the exception of the 
Outer shell of the plagioclase phenocrysts, as small interstitial grains. Analcite is 
present in phenocrystic individuals which in thin section have an approach to 
square or hexagonal outline. It also occurs as rounded blebs, sometimes inclosing 
or penetrated by prisms of pyroxene, and as irregular interstitial grains. A com¬ 
mon alteration is to radiating needles of stilbite. Augite occurs as phenocrysts, 
generally characterized by good crystallographic form, and also as more or less 
irregular grains in the groundmass. Cleavage is noticeably developed. Besides 
the usual prismatic cleavage, there is a prominent cleavage parallel to the clino- 
pinacoid, and sometimes ortliopinacoidal and negative orthodomal cleavage. The 
phenocrysts not infrequently show twinning parallel to the orthopinacoid and zonal 
structure is almost always present. Small particles of glass are sometimes included 
near the periphery of the larger individuals. In one rock, at least, that from a dike 
near the Gold Bond mine, corrosion and subsequent enlargement of the pyroxene 
individuals have taken place. The mineral is usually colorless or very light green. 
In one or two instances it shows a faint purple tint, indicative of the presence of 
titania, and then possesses a slight pleochroism. Olivine occurs as large and small 
phenocrysts, more or less idiomorphic, and sometimes as small graips in the ground- 
mass. When fresh, it shows the usual irregular cracks along which alteration begins 
and in certain instances shows an imperfect cleavage also. Generally it is in an 
advanced stage of alteration, the more common and noticeable products being car¬ 
bonates, serpentine, talc, and a material corresponding to what has been called 
iddingsite. ° Biotite occurs rarely as phenocrysts, but is comparatively abundant 
in the groundmass as small grains, usually reddish brown, intensely pleochroic, and 
with high interference colors. It sometimes shows both corrosion and resorption. 
Hornblende occurs sparingly as irregular individuals of brown color. A dike in the 
Ajax mine contains a weakly pleochroic bluish-green amphibole. The apatite, 
which occurs in prisms as much as 2 mm. in cross section, is particularly free from 
inclusions. 
In two of the rocks of this type a few' grains of obviously secondary quartz are 
found. Epidote is rather plentiful in some of the partially decomposed specimens 
and carbonates are nearly always abundant. 
These rocks possess a texture which is about midway between trachytic and 
intersertal. Most of them appear to be holocrystalline, but some undoubtedly 
contain small areas of brown, turbid glass. 
A chemical analysis of material from the Isabella dike w 7 as made by W. T. 
Schaller, and serves to show the alkalic nature of the rock and its close relation to 
the trachydolerite of the Bull Cliff mass. 
a Lawson, A. C., Bull. Dept. Geol. Univ. California, vol. 1, p. 31. 
