78 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
Mr. Cross. Microscopical and chemical study indicate that, though the rock is 
closely related to the latite-phonolites, it does not conform to the type as do the 
other varities, and the differences are sufficient to put it with the trachytes: it is 
called a biotite trachyte. The Battle Mountain mass appears to be an intrusion 
distinct from the latite-phonolites, but many of the latter contain biotite in amounts 
reaching considerable importance. 
This biotite trachyte contains numerous phenocrysts of feldspar, orthoclase, 
albite, microperthite, and oligoclase or oligoclase-albite being present in decreasing 
amount. Biotite is abundant in scales 1 or 2 mm. across, which are sometimes bent 
and frequently show the common magmatic alteration into augite and magnetite. 
As has been shown on page 74, from the chemical character of the rock this mica 
must be an iron-rich species, probably related to lepidomelane. Phenocrysts of 
augite are occasionally present. Apatite and sphene are rather common in crystals 
of more than ordinary size and grains of magnetite are numerous. Small inter¬ 
stitial grains of quartz occur sparingly in the Mineral Hill rock. Very little analcite, 
sodalite, or nosean has been detected in most specimens. The groundmass is 
imperfectly trachytoidal and consists almost wholly of orthoclase. The chemical 
character of this rock, showing its trachytic nature and its relation to the latite- 
phonolites, is expressed in an analysis given with those of the latite-phonolites on 
the next page. 
TEXTURE. 
These rocks all contain a more or less important groundmass made up of feld¬ 
spar and pyroxene microlites and grains and of particles of iron ore, with sometimes 
plentiful small crystalline grains of sodalite. As in the phonolites, the development 
of the feldspars controls the character of the texture. The most common form is as 
small laths with parallel arrangement showing beautifully the flow movements of 
the magma and giving a trachytic texture similar to that of the phonolites. Like- 
wi e a less perfect crystallographic development of the groundmass feldspars leads 
to a microgranular texture, though fluidal structure can usually be distinguished. 
Abundant development of feldspar in the form of phenocrysts leaves the groundmass 
very subordinate, and in that case it is usually microgranular with little or no 
indication of a flow structure. The very small amount of glass included in some 
of the feldspar phenocrysts is hardly sufficient to warrant classing these rocks as 
other than holocrystalline porphyritic. The principal textural differences between 
the latite-phonolites and the phonolites proper are the greater tendency of the former 
toward phanerocrystalline development, the somewhat coarser grain of their 
groundmass, and their more frequent and decided deviation from the characteristic 
trachytic texture. 
