84 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
* quite aphanitic, and in appearance resembles rather the phonolites than the latite- 
phonolites. Microscopically it is characterized by a number of small lath-shaped 
feldspar phenocrysts, considerable segirine, aegirine-augite, and blue amphibole, 
and irregular, interstitial masses of analcite, in a well-defined trachytic ground- 
mass. Between the feldspar microlites occur numerous grains of sodalite. Nephe- 
line, however, could not be identified with certainty, though a number of minute 
partially decomposed grains of higher index of refraction than the feldspar roughly 
correspond in outline to nepheline. The aegirine occurs in its usual tabular habit, 
but the aegirine-augite takes the more prismatic form of the ordinary pyroxenes. 
The habit of the pyroxene and the scarceness or entire absence of nepheline places 
this rock between the two main groups, but somewhat nearer to phonolite than to 
latite-phonolite. 
The third variety occurs south of the main street of Altman, just west of the 
Deadwood mine. It forms part of a large area of latite-phonolite and appears 
to be identical with the finer grained varieties of that group. Besides numerous feld¬ 
spar tablets and laths of good size, it contains large and small nosean crystals, clear 
little grains and ragged patches of pure aegirine, phenocrysts of aegirine-augite and 
augite, large grains of titanite, abundant sodalite microlites, and small shreds of 
the unknown mineral of high index and double refraction found in the phonolites, 
all contained in a comparatively coarse-grained trachytic groundmass. One or two 
rectangular sections of a partially decomposed mineral may be nepheline. 
It is thus seen that though the phonolites are a particularly well-characterized 
group and the latite-phonolites are fairly well defined, a few intervening members 
reveal the close relation of the two and unite them all into one rock series. 
SYENITE. 
GENERAL DESCRIPTION. 
Granular rocks closely resembling and connected by textural gradations with 
the latite-phonolites occur in various parts of the volcanic area. The larger masses 
were mapped by Cross and called by him nepheline syenite. In the recent study, 
however, no nepheline has been found. Cross’s statement of the presence of this 
mineral is not very conclusive and seems to have been based mainly on the presence 
of decomposition products supposed to have been derived from nepheline. 01 These 
syenites are alkali-rich rocks and are most closely related to the alkali syenites of 
Iiosenbusch; but because of too high lime and magnesia and rather low silica, they 
are in the following description classed broadly as syenites and confined to certain 
characteristics. They show certain affinities to the essexites, but differ in being 
decidedly lower in iron, magnesia, and lime; richer in alkalies, particularly potash; 
and somewhat higher in silica and alumina. They have lower bivalvent bases and 
higher alkalies than the monzonites. While a little more basic than their porphy- 
ritic equivalents, the latite-phonolites, exemplifying a common difference between 
the granular and the porphyritic rocks, 6 they nevertheless show certain close rela¬ 
tionships not only to that rock type, but to the phonolites as well. On the other 
a General geology of the Cripple Creek district, Colorado: Sixteenth Ann. Rept. U. S. Geol. Survey, pt. 2, 1895, p. 44. 
i>Cf. Rosenbusch. H., Elemente der Gesteinslehre, Stuttgart, 1901, p. 249. 
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