TERTIARY VOLCANIC ROCKS-LEUCITOPHYRE. 67 
A calculation of the average analysis, No. V, gives a good idea of the relative 
amounts of the various minerals present: 
Mineral composition of phonolite. 
Soda orthoclase. 51.19 
Nepheline. 11.27 
Analcite. 11.41 
Nosean.89 
' Sodalite... 4. 25 
iEgirine. 4.16 
Augite. 2. 64 
Apatite.09 
Titanite.£1 
Magnetite.48 
Other minerals, hygroscopic water, water of 
hydration, etc. 3.01 
100.00 
This computation is complicated by the presence in the rocks of kaolin, a little 
sericite, and perhaps a small amount of zeolites, resulting largely from the feldspar. 
Careful examination of thin sections of the specimens analyzed led to the conclusion 
that not more than 0.75 per cent H,0 on the average has been introduced by the 
hydration of original minerals. 
The ratio of K 2 0 to Na 2 0 in the soda-orthoclase was assumed to be 33.57:17.32, 
or approximately 2:1. 
The nepheline was considered free from potash. 
The following composition was assumed for augite: CaO, MgO, FeO (Al,Fe) 
20 3 .3 Si0 2 , Al:Fe" ' being 4:1. 
It is probable that the titania is not present wholly as titanite, but occurs 
mainly in other minerals. 
The zirconia forms very little zircon and probably is mostly united in other 
minerals. 
SUMMARY. 
From the preceding discussion it is obvious-that the phonolites of the Cripple 
Creek district are a particularly well-defined group of rocks, and that chemically, 
mineralogically, and texturally they are typical of the recognized phonolite family. 
LEUCITOPHYRE (?). 
In examining a thin section of a specimen of breccia from the dump of a shaft 
about a quarter of a mile north of the Mint mine, on Gold Hill, there was observed 
a small fragment of a rock which has not been seen elsewhere in the district. The 
hand specimen shows a fairly coarse breccia with much granite and latite-phonolite 
and two fragments of a fine-grained, purplish-brown rock, which holds an occasional 
small phenocryst of feldspar and in which the hand lens reveals, in addition, abundant 
minute whitish specks evenly distributed through the mass. Under the microscope 
the rock shows considerable alteration. The feldspar phenocrysts have a patchy, 
uneven extinction and are either microperthite or anorthoclase. The most con¬ 
spicuous constituent is the mineral observed by the hand lens. It forms crystals 
of sometimes square and sometimes hexagonal, but usually octagonal, outline, 
which attain a maximum size of 0.5 mm. The original material is decomposed 
into a zeolite of weak refractive power, and low aggregate polarization. The 
mineral can have been only leucite or analcite. In those rocks which contain 
phenocrysts of analcite, the analcite is seldom abundant and generally occurs in 
