188 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
Fluorite is rarely ver} r abundant, but is widely distributed. It lias not the same 
tendency to crystallographic development as in the filled veins, but occurs in little 
masses filling small cavities of dissolution or replacing any one of the constituents 
of the rock. The color is always purple, frequently deepened in spots. 
Rutile forms easily from titanite; pseudomorphs of rutile, calcite, and chal¬ 
cedony after crystals of titanite are common in the latite-phonolites. 
METASOMATIC CHANGES IN PHONOLITE. 
The changes which have taken place are rarely very far-reaching. A phonolite 
dike, followed by the vein, on the 370-foot level in the C. K. & N mine shows small 
cavities of dissolution coated by quartz, pyrite, fluorite, and tellurides. Under 
the microscope the structure of the rock is seen to be perfectly preserved. The 
segirine-augite is replaced b}^ calcite and nests of serpentine; pyrite is abundant in 
small crystals, in many places probably replacing magnetite. There is very little 
sericite, and the orthoclase seems practically fresh. This phonolite is a low- 
grade ore. 
A fine-grained dike from level 10 of the Last Dollar mine, 100 feet south of 
the shaft, shows dolomite or calcite distributed throughout the rock; here also 
magnetite is converted to pyrite and siderite. Small pyrite crystals are abundant 
and veinlets of adularia, carbonates, pyrite, zinc blende, and galena traverse the 
rock; the adularia is the earliest mineral formed. This altered dike is not classed 
as ore. A light yellowish-gray phonolite from the Cheyenne vein on level 11 of 
the Isabella mine, 800 feet south of the Lee shaft, is a normal rock containing very 
little pyrite and some fluorite in irregular aggregates. The nepheline is converted 
to sericite, while the orthoclase appears unaltered. However, many of these rocks 
may contain much more adularia than is apparent. 
METASOMATIC CHANGES IN SYENITE. 
The normal sj^enite from the Last Dollar mine contains no carbonates and, 
sericite, green mica, and chlorite are only slightly developed. The magnetite is 
not altered. The rock along the vein at the north end of level 11 in the same 
mine is a syenite which has been subjected to unusually strong metasomatic action. 
It is bleached and softened and contains much pyrite, besides some black zinc 
blende and fluorite. The latter three minerals, with much quartz and adularia, 
form irregularly outlined replacement veins. Lmder the microscope the most 
abundant constituent is a granular orthoclase, while the ferromagnesian silicates 
and the magnetite have disappeared. Dolomite in anhedrons and rliombohedral 
forms mixed with a little sericite replaces this orthoclase abundantly. Sharply 
defined cubes and pyritohedrons of pyrite are plentiful, together with grains of 
black zinc blende. Rims of adularia sometimes surround the pyrite. Cavities of 
dissolution contain nests of quartz, adularia, and fluorspar; veinlets, probably 
chiefly formed bj" replacement, contain carbonates, quartz, and adularia, and in 
one place calaverite with tetraliedrite; tellurides do not, however, appear in 
the mass of the rock. 
