190 GEOLOGY AND GOLD DEPOSITS OF THE CRIPPLE CREEK DISTRICT. 
Under B is given, for comparison, an analysis of a latite-phonolite poor in lime, 
which probably best corresponds to the composition of the original rock of A. This 
is, of course, an assumption, but the chemical composition of the Cripple Creek rocks 
is so well known that it can not be far from the truth. There are no volcanic rocks 
known from the district in which potassium predominates in the manner shown by A. 
The principal change, then, consists in a transformation of sodic silicates to potassic 
(serieite and adularia), and in the introduction of sulphur and fluorine. There is 
probably a slight loss of silica and a gain of water, though the fresh rocks usually 
contain 1 or 2 per cent chemically combined water which may be applied to the 
formation of secondary serieite. There is also a loss of chlorine and sulphuric acid 
from sodalite and hauynite, and a loss of a part of the lime, while magnesia, iron 
baryta, and titania have remained about constant. 
The rocks from the Golden Cycle mine are not conspicuously altered, except for 
small and abundant sharply defined crystals of pyrite; sometimes they are slightly 
bleached. The pyroxene crystals are converted to serieite or carbonates, or both, 
but chlorite and serpentine are generally absent. Veinlets of adularia intersect 
the rocks. With the maximum of alteration perhaps 10 per cent of dolomite and 10 
per cent of serieite have developed in the rock. The orthoclase contains fibers 
of serieite along the cleavage planes and sometimes irregularly replacing dolomite; 
very rarely, however, is the entire crystal replaced. Magnetite alters into pyrite. 
In the latite-phonolite of the Vindicator, Last Dollar, American Eagle, and 
Gold Sovereign mines the alteration is along the same lines, although dolomitic 
carbonates with a decided tendency to crystallization are abundant and replace both 
feldspars and pyroxenes. The amount of new-formed minerals rarely exceeds 15 per 
cent. In the latite-phonolite from the Last Dollar mine it is again noted that the 
small sodalite crystals first become sericitized. Pyroxene is converted to dolomite, 
with a little fluorite, serpentine, and pyrite (PI. XVII, D ). The large rounded 
magnetite grains are surrounded by white rings of siderite and the remaining part is 
more or less completely converted to pyrite. Titanite alters into marginal rutile 
with centers of calcite and chalcedony (PI. XVII, E .) A latite-phonolite from the 
dump of the American Eagle shaft shows abundant cavities of dissolution Idled 
with adularia, pyrite, rutile, and serieite. In one case tufted serieite coats the walls 
of the rounded cavity, which is filled with adularia. 
A latite-phonolite from the 1,300-foot level of the mine is a greenish-gray rock 
with crusted veinlets of liglit-greenish dolomite containing calaverite; this is sur¬ 
mounted by fluorite crystals covered again by clear hyalitic silica. The metasomatic 
alteration is slight. Pyroxene crystals are converted to serpentine and chlorite 
with magnetite, and the latter, near the veinlets, alters to pyrite. Crystals of 
pyrite are distributed through the ground mass and serieite and adularia develop 
in the orthoclase, the adularia usually filling cavities of dissolution. 
Another specimen from a very wide and rich part of No. 3 vein in the Vindi¬ 
cator mine, on level 11, is a greenish-gray latite-phonolite, in which some orthoclase 
crystals are visible. It contains much finely divided pyrite and seams of molyb¬ 
denite and calaverite. While the seams are rich, the rock between them is considered 
as barren. The microscope shows a large amount of orthoclase, both as phenocrysts 
and microliths in the groundmass. Serieite and carbonate occur in moderate quantity 
